CHAPTER IV.. The Carboniferous Flora--Culmination of the

THE CARBONIFEROUS FLORA--CULMINATION OF THE ACROGENS--FORMATION OF COAL.

Ascending from the Erian to the Carboniferous system, so called because it contains the greatest deposits of anthracite and bituminous coal, we are still within the limits of the Palæozoic period. We are still within the reign of the gigantic club-mosses, cordaites, and taxine pines. At the close of the Erian there had been over the whole northern hemisphere great changes of level, accompanied by active volcanic phenomena, and under these influences the land flora seems to have much diminished. At length all the old Erian species had become extinct, and their place was supplied by a meagre group of lycopods, ferns, and pines of different species from those of the preceding Erian. This is the flora of the Lower Carboniferous series, the Tweedian of England, the Horton series of Nova Scotia, the lower coal-measures of Virginia, the culm of Germany. But the land again subsided, and the period of the marine limestone of the Lower Carboniferous was introduced. In this the older flora disappeared, and when the land emerged we find it covered with the rich flora of the coal-formation proper, in which the great tribes of the lycopods and cordaites attained their maxima, and the ferns were continued as before, though under new generic and specific forms.

There is something very striking in this succession of a new plant world without any material advance. It is like passing in the modern world from one district to another, in which we see the same forms of life, only represented by distinct though allied species. Thus, when the voyager crosses the Atlantic from Europe to America, he meets with pines, oaks, birches, poplars, and beeches of the same genera with those he had left behind; but the species are distinct. It is something like this that meets us in our ascent into the Carboniferous world of plants. Yet we know that this is a succession in time, that all our old Erian friends are dead and buried long ago, and that these are new forms lately introduced (Fig. 32).

Conveying ourselves, then, in imagination forward to the time when our greatest accumulations of coal were formed, and fancying that we are introduced to the American or European continent of that period, we find ourselves in a new and strange world. In the Devonian age, and even in the succeeding Lower Carboniferous, there was in the interior of America a wide inland sea, with forest belts clinging to its sides or clothing its islands. But in the coal period this inland sea had given place to vast swampy flats, and which, instead of the oil-bearing shales of the Erian, were destined to produce those immense and wide-spread accumulations of vegetable matter which constitute our present beds of bituminous and anthracite coal. The atmosphere of these great swamps is moist and warm. Their vegetation is most exuberant, but of forms unfamiliar to modern eyes, and they swarm with insects, millipedes, and scorpions, and with batrachian reptiles large and small, among which we look in vain for representatives of the birds and beasts of the present day.

Prominent among the more gigantic trees of these swampy forests are those known to us as Sigillariæ (Fig. 33). They have tall, pillar-like trunks, often several feet in diameter, ribbed like fluted columns, but in the reverse way, and spreading at the top into a few thick branches, which are clothed with long, grass-like leaves. They resemble in some respects the Lepidodendra of the Erian age, but are more massive, with ribbed instead of scaly trunks, and longer leaves. If we approach one of them more closely, we are struck with the regular ribs of its trunk, dotted with rows of scars of fallen leaves, from which it receives its name Sigillaria, or seal-tree (Figs. 34-37). If we cut into its stem, we find that, instead of the thin bark and firm wood with which we are familiar in our modern trees, it has a hard external rind, then a great thickness of cellular matter with rope-like bands of fibres, constituting an inner bark, while in the centre is a firm, woody axis of comparatively small diameter, and somewhat intermediate in its structures between that of the Lepidodendra and those of the cycads and the taxine conifers. Thus a great stem, five feet in diameter, may consist principally of cellular and bast fibres with very little true woody matter. The roots of this tree are perhaps its most singular feature. They usually start from the stem in four main branches, then regularly bifurcate several times, and then run out into great cylindrical cables, running for a long distance, and evidently intended to anchor the plant firmly in a soft and oozy soil. They were furnished with long, cylindrical rootlets placed regularly in a spiral manner, and so articulated that when they dropped off they left regular rounded scars. They are, in short, the Stigmariæ, which we have already met with in the Erian (Figs. 38, 39). In Fig. 33 I have endeavoured to restore these strange trees. It is not wonderful that such plants have caused much botanical controversy. It was long before botanists could be convinced that their roots are properly roots at all, and not stems of some aquatic plant. Then the structure of their stems is most puzzling, and their fruit is an enigma, for while some have found connected with them cones supposed to resemble those of lycopods, others attribute to them fruits like those of yew-trees. For years I have been myself gathering materials from the rich coal-formation deposits of Nova Scotia in aid of the solution of these questions, and in the mean time Dr. Williamson, of Manchester, and Renault and other botanists in France, have been amassing and studying stores of specimens, and it is still uncertain who may finally be the fortunate discoverer to set all controversies at rest. My present belief is, that the true solution consists in the fact that there are many kinds of Sigillariæ. While in the modern forests of America and Europe the species of any of our ordinary trees, as oaks, birches, or maples, may almost be counted on one's fingers, Schimper in his vegetable palæontology enumerates about eighty species of Carboniferous Sigillariæ; and while on the one hand many of these are so imperfectly known that they may be regarded as uncertain, on the other hand many species must yet remain to be discovered. Now, in so vast a number of species there must be a great range of organisation, and, indeed, it has already been attempted to subdivide them into several generic groups. The present state of the question appears to me to be this, that in these Sigillariæ we have a group divisible into several forms, some of which will eventually be classed with the Lepidodendra as lycopods, while others will be found to be naked-seeded phænogams, allied to the pines and cycads, and to a remarkable group of trees known as Cordaites, which we must shortly notice.

In a recent memoir (Berlin, 1887) Stur has raised the number of species in one subdivision of the Sigillariæ (the Favulariæ) to forty-seven!

Before considering other forms of Carboniferous vegetation, let us glance at the accumulation of coal, and the agency of the forests of Sigillariæ therein. Let us imagine, in the first instance, such trees as those represented in the figures, growing thickly together over vast swampy flats, with quantities of undergrowth of ferns and other plants beneath their shade, and accumulating from age to age in a moist soil and climate a vast thickness of vegetable mould and trunks of trees, and spores and spore-cases, and we have the conditions necessary for the growth of coal. Many years ago it was observed by Sir William Logan that in the coal-field of South Wales it was the rule with rare exceptions that, under every bed of coal, there is a bed of clay filled with roots of the Stigmaria, already referred to as the root of Sigillaria. This discovery has since been extended to all the coal-fields of Europe and America, and it is a perfectly conclusive fact as regards the origin of coal. Each of these "under-clays," as they are called, must, in fact, have been a soil on which grew, in the first instance, Sigillariæ and other trees having stigmaria-roots. Thus, the growth of a forest of Sigillariæ was the first step toward the accumulation of a bed of coal. More than this, in some of the coarser and more impure coals, where there has been sufficient earthy matter to separate and preserve impressions of vegetable forms, we can see that the mass of the coal is made up of flattened Sigillariæ, mixed with vegetable débris of all kinds, including sometimes vast quantities of lepidodendroid spores, and the microscopic study of the coal gives similar results (Fig. 40). Further, on the surfaces of many coals, and penetrating the shales or sandstones which form their roofs, we find erect stumps of sigillaria and other trees, showing that the accumulation of the coal terminated as it had begun, by a forest-growth. I introduce here a section of a few of the numerous beds of coal exposed in the cliffs of the South Joggins, in Nova Scotia, in illustration of these facts. We can thus see how in the slowly subsiding areas of the coal-swamps successive beds of coal were accumulated, alternating with beds of sandstone and shale (Figs. 41, 42). For other details of this kind I must refer to papers mentioned in the sequel.

Returning to the more special subject of this work, I may remark that the lepidodendroid trees and the ferns, both the arborescent and herbaceous kinds, are even more richly represented in the Carboniferous than in the preceding Erian, I must, however, content myself with merely introducing a few representatives of some of the more common kinds, in an appended note, and here give a figure of a well-known Lower Carboniferous lepidodendron, with its various forms of leaf-bases, and its foliage and fruit (Fig. 43), and a similar illustration of an allied generic form, that known as Lepidophloios (Fig. 44).

For full descriptions of these, see "Acadian Geology."

Another group which claims our attention is that of the Calamites. These are tall, cylindrical, branchless stems, with whorls of branchlets, bearing needle-like leaves and spreading in stools from the base, so as to form dense thickets, like Southern cane-brakes (Fig. 46). They bear, in habit of growth and fructification, a close relation to our modern equisetums, or mare's-tails, but, as in other cases we have met with, are of gigantic size and comparatively complex structure. Their stems, in cross-section, show radiating bundles of fibres, like those of exogenous woods, yet the whole plan of structure presents some curious resemblances to the stems of their humble successors, the modern mare's-tails. It would seem, from the manner in which dense brakes of these Calamites have been preserved in the coal-formation of Nova Scotia, that they spread over low and occasionally inundated flats, and formed fringes on the seaward sides of the great Sigillaria forests. In this way they no doubt contributed to prevent the invasion of the areas of coal accumulation by the muddy waters of inundations, and thus, though they may not have furnished much of the material of coal, they no doubt contributed to its purity. Many beautiful plants of the genera Asterophyllites and Annularia are supposed to have been allied to the Calamites, or to have connected them with the Rhizocarps. The stems and fruit of these plants have strong points of resemblance to those of Sphenophyllum, and the leaves are broad, and not narrow and angular like those of the true Calamites (Fig. 45).

No one has done more than my friend Dr. Williamson, of Manchester, to illustrate the structure of Calamites, and he has shown that these plants, like other cryptogams of the Carboniferous, had mostly stems with regular fibrous wedges, like those of exogens. The structure of the stem is, indeed, so complex, and differs so much in different stages of growth, and different states of preservation, that we are in danger of falling into the greatest confusion in classifying these plants. Sometimes what we call a Calamite is a mere cast of its pith showing longitudinal striæ and constrictions at the nodes. Sometimes we have the form of the outer surface of the woody cylinder, showing longitudinal ribs, nodes, and marks of the emission of the branchlets. Sometimes we have the outer surface of the plant covered with a smooth bark showing flat ribs, or almost smooth, and having at the nodes regular articulations with the bases of the verticillate branchlets, or on the lower part of the stem the marks of the attachment of the roots. The Calamites grew in dense clumps, budding off from one another, sometimes at different levels, as the mud or sand accumulated about their stems, and in some species there were creeping rhizomata or root-stocks (Figs. 46 to 49).

But all Calamites were not alike in structure. In a recent paper Dr. Williamson describes three distinct structural types. What he regards as typical Calamites has in its woody zone wedges of barred vessels, with thick bands of cellular tissue separating them. A second type, which he refers to Calamopitus, has woody bundles composed of reticulated or multiporous fibres, with their porous sides parallel to the medullary rays, which are better developed than in the previous form. The intervening cellular masses are composed of elongated cells. This is a decided advance in structure, and is of the type of those forms having the most woody and largest stems, which Brongniart named Calamodendron (Fig. 50). A third form, to which Dr. Williamson seems to prefer to assign this last name, has the tissue of the woody wedges barred, as in the first, but the medullary rays are better developed than in the second. In this third form the intermediate tissue, or primary medullary rays, is truly fibrous, and with secondary medullary rays traversing it. My own observations lead me to infer that there was a fourth type of calamitean stem, less endowed with woody matter, and having a larger fistulous or cellular cavity than any of those described by Dr. Williamson.

"Memoirs of the Philosophical Society," Manchester, 1886-'87.

There is every reason to believe that all these various and complicated stems belonged to higher and nobler types of mare's-tails than those of the modern world, and that their fructification was equisetaceous and of the form known as Calamostachys.

We have already seen that noble tree-ferns existed in the Erian period, and these were continued, and their number and variety greatly extended, in the Carboniferous. In regard to the structure of their stems, and the method of supporting these by aërial roots, the tree-ferns of all ages have been nearly alike, and the form and structure of the leaves, except in some comparatively rare and exceptional types, has also been much the same. Any ordinary observer examining a collection of coal-formation ferns recognises at once their kinship to the familiar brackens of our own time. Their fructification is, unfortunately, rarely preserved, so that we are not able, in the case of many species, to speak confidently of their affinities with modern forms; but the knowledge of this subject has been constantly extending, and a sufficient amount of information has been obtained to enable us to say something as to their probable relationships. (Figs. 51 to 55.)

The families into which modern ferns are divided are, it must be confessed, somewhat artificial, and in the case of fossil ferns, in which the fructification is for the most part wanting, it is still more so, depending in great part on the form and venation of the divisions of the fronds. Of about eight families into which modern ferns are divided, seven are found in a fossil state, and of these, four at least, the Cyathaceæ, the Ophioglosseæ, the Hymenophyllaceæ, and the Marattiaceæ, go back to the coal-formation.

Mr. R. Kidston has recently described very interesting forms of fern fructification from the coal-formation of Great Britain, and much has been done by European palæobotanists, and also by Lesquereux and Fontaine in America.

Some of these ferns have the more complex kind of spore-case, with a jointed, elastic ring. It is to be observed, however, that those forms which have a simple spore-case, either netted or membranous, and without annulus, are most common in the Devonian and lowest Carboniferous. Some of the forms in these old rocks are somewhat difficult to place in the system. Of these, the species of Archæopteris, of the Upper and Middle Erian, are eminent as examples. This type, however, scarcely extends as high as the coal-formation. Some of the tree-ferns of the Carboniferous present very remarkable features. One of these, of the genus Megaphyton, seems to have two rows of great leaves, one at each side of the stem, which was probably sustained by large bundles of aërial roots (Fig. 56).

The pretty little ferns of the genus Botrychium (moonwort), so common in American and European woods, seem to be their nearest modern allies.

In the Carboniferous, as in the Erian, there are leaves which have been referred to ferns, but are subject to doubt, as possibly belonging to broad-leaved taxine trees allied to the gingko-tree of China. One of these, represented in Fig. 57, has been found in the coal-formation of Nova Scotia, and referred to the doubtful genus Noeggerathia. Fontaine has proposed for similar leaves found in Virginia the new generic name Saportea.

Ferns, as might be inferred from their great age, are at the present time dispersed over the whole world; but their headquarters, and the regions to which tree-ferns are confined, are the more moist climates of the tropics and of the southern hemisphere. The coal-swamps of the northern hemisphere seem to have excelled even these favoured regions of the present world as a paradise for ferns.

I have already stated that the Carboniferous constitutes the headquarters of the Cordaites (Fig. 58), of which a large number of species have been described, both in Europe and America. We sometimes, though rarely, find their stems showing structure. In this case we have a large cellular pith, often divided by horizontal partitions into flat chambers, and constituting the objects which, when detached, are called Sternbergiæ (Fig. 62). These Sternbergia piths, however, occur in true conifers as well, as they do in the modern world in some trees, like our common butternut, of higher type; and I showed many years ago that the Sternbergia type may be detected in the young twigs of the balsam-fir (Abies balsamifera). The pith was surrounded by a ring of scalariform or barred tissue, often of considerable thickness, and in young stems so important as to have suggested lycopodiaceous affinities. But as the stem grew in size, a regular ring of woody wedges, with tissue having rounded or hexagonal pores or discs, like those of pines, was developed. Outside this was a bark, often apparently of some thickness. This structure in many important points resembles that of cycads, and also approaches to the structure of Sigillaria, while in its more highly developed forms it approximates to that of the conifers.

On the stems so constructed were placed long and often broad many-nerved leaves, with rows of stomata or breathing-pores, and attached by somewhat broad bases to the stem and branches. The fruit consisted of racemes, or clusters of nutlets, which seem to have been provided with broad lateral wings for flotation in the air, or in some cases with a pulpy envelope, which flattens into a film. There seem to have been structures of both these kinds, though in the state of preservation of these curious seeds it is extremely difficult to distinguish them. In the first case they must have been intended for dissemination by the wind, like the seeds of spruces. In the latter case they may have been disseminated like the fruits of taxine trees by the agency of animals, though what these were it would be difficult to guess. These trees had very great reproductive power, since they produced numerous seeds, not singly or a few together, as in modern yews, but in long spikes or catkins bearing many seeds (Fig. 59).

It is to be observed that the Cordaites, or the Cordaitinæ, as they have been called, as a family, constitute another of those intermediate groups with which we have already become familiar. On the one hand they approach closely to the broader-leaved yews like Gingko, Phyllocladus, and Podocarpus, and, on the other hand, they have affinities with Cycadaceæ, and even with Sigillariæ. They were beautiful and symmetrical trees, adding something to the variety of the rather monotonous Palæozoic forests. They contributed also somewhat to the accumulation of coal. I have found that some thin beds are almost entirely composed of their leaves, and the tissues of their wood are not infrequent in the mineral charcoal of the larger coal-seams. There is no evidence that their roots were of the stigmaroid type, though they evidently grew in the same swampy flats with the Sigillariæ and Calamites.

Engler; Cordaitées of Renault.

It may, perhaps, be well to say here that I believe there was a considerably wide range of organisation in the Cordaitinæ as well as in the Calamites and Sigillariæ, and that it will eventually be found that there were three lines of connection between the higher cryptogams and the phænogams, one leading from the lycopods by the Sigillariæ, another leading by the Cordaites, and the third leading from the Equisetums by the Calamites. Still further back the characters afterward separated in the club-mosses, mare's-tails, and ferns, were united in the Rhizocarps, or, as some now, but I think somewhat unreasonably, prefer to call them, the "heterosporous Filicinæ." In the more modern world, all the connecting links have become extinct and the phænogams stand widely separated from the higher cryptogams. I do not make these remarks in a Darwinian sense, but merely to state what appear to be the lines of natural affinity and the links wanting to give unity to the system of nature.

Of all the trees of the modern world, none are perhaps so widely distributed as the pines and their allies. On mountain-tops and within the Arctic zone, the last trees that can struggle against the unfavourable conditions of existence are the spruces and firs, and in the warm and moist islands of the tropics they seem equally at home with the tree-ferns and the palms. We have already seen that they are a very ancient family, and in the sandstones of the coal-formation their great trunks are frequently found, infiltrated with calcareous or silicious matter, and still retaining their structure in the greatest perfection (Fig. 60). So far as we know, the foliage of some of them which constitutes the genera Walchia and Araucarites of some authors (Figs. 60, 63) was not dissimilar from that of modern yews and spruces, though there is reason to believe that some others had broad, fern-like leaves like those of the gingko. None of them, so far as yet certainly known, were cone-bearing trees, their fruit having probably been similar to that of the yews (Fig. 61). The minute structures of their stems are nearer to those of the conifers of the islands of the southern hemisphere than to that of those in our northern climes--a correlation, no doubt, to the equable climate of the period. There is not much evidence that they grew with the Sigillariæ in the true coal-swamps, though some specimens have been found in this association. It is more likely that they were in the main inland and upland trees, and that in consequence they are mostly known to us by drifted trunks borne by river inundations into the seas and estuaries.

A remarkable fact in connection with them, and showing also the manner in which the most durable vegetable structures may perish by decay, is that, like the Cordaites, they had large piths with transverse partitions, a structure which, as I have already mentioned, appears on a minute scale in the twigs of the fir-tree, and that sometimes casts of these piths in sandstone appear in a separate form, constituting what have been named Sternbergiæ or Artisiæ. As Renault well remarks with reference to Cordaites, the existence of this chambered form of pith implies rapid elongation of the stem, so that the Cordaites and conifers of the coal-formation were probably quickly growing trees (Fig. 62).

The same general statements may be made as to the coal-vegetation as in relation to that of the Erian. In the coal period we have found none of the higher exogens, and there are only obscure and uncertain indications of the presence of endogens, which we may reserve for a future chapter; but gymnosperms abound and are highly characteristic. On the other hand, we have no mosses or lichens, and very few Algæ, but a great number of ferns and Lycopodiaceæ or club-mosses (Fig. 63). Thus, the coal-formation period is botanically a meeting-place of the lower phænogams and the higher cryptogams, and presents many forms which, when imperfectly known, have puzzled botanists in regard to their position in one or other series. In the present world, the flora most akin to that of the coal period is that of warm, temperate regions in the southern hemisphere. It is not properly a tropical flora, nor is it the flora of a cold region, but rather indicative of a moist and equable climate. Still, we must bear in mind that we may often be mistaken in reasoning as to the temperature required by extinct species of plants, differing from those now in existence. Further, we must not assume that the climatal conditions of the northern hemisphere were in the coal period at all similar to those which now prevail. As Sir Charles Lyell has shown, a less amount of land in the higher latitudes would greatly modify climates, and there is every reason to believe that in the coal period there was less land than now. Further, it has been shown by Tyndall that a very small additional amount of carbonic acid in the atmosphere would, by obstructing the radiation of heat from the earth, produce almost the effect of a glass roof or conservatory, extending over the whole world. Again, there is much in the structure of the leaves of the coal-plants, as well as in the vast amount of carbon which they accumulated in the form of coal, and the characteristics of the animal life of the period, to indicate, on independent grounds, that the carboniferous atmosphere differed from that of the present world in this way, or in the presence of more carbonic acid--a substance now existing in the very minute proportion of one thousandth of the whole--a quantity adapted to the present requirements of vegetable and animal life, but probably not to those of the coal period.

Thus, if we inquire as to any analogous distribution of plants in the modern world, we find this only in the warmer insular climates of the southern hemisphere, where ferns, lycopods, and pines appear under forms somewhat akin to those of the Carboniferous, but mixed with other types, some of which are modern, others allied to those of the next succeeding geological ages of the Mesozoic and Tertiary; and under these periods it will be more convenient to make comparisons.

The readers of recent English popular works on geology will have observed the statement reiterated that a large proportion of the material of the great beds of bituminous coal is composed of the spore-cases of lycopodiaceous plants--a statement quite contrary to that resulting from my microscopical examinations of the coal of more than eighty coal-beds in Nova Scotia and Cape Breton, as stated in "Acadian Geology" (page 463), and more fully in my memoir of 1858 on the "Structures in Coal," and that of 1866, on the "Conditions of Accumulation of Coal." The reason of this mistake is, that an eminent English naturalist, happening to find in certain specimens of English coal a great quantity of remains of spores and spore-cases, though even in his specimens they constitute only a small portion of the mass, and being apparently unacquainted with what others had done in this field, wrote a popular article for the "Contemporary Review," in which he extended an isolated and exceptional fact to all coals, and placed this supposed origin of coal in a light so brilliant and attractive that he has been followed by many recent writers. The fact is, as stated in "Acadian Geology," that trunks of Sigillariæ and similar trees constitute a great part of the denser portion of the coal, and that the cortical tissues of these rather than the wood remain as coal. But cortical or epidermal tissues in general, whether those of spore-cases or other parts of plants, are those which from their resistance to water-soakage and to decay, and from their highly carbonaceous character, are best suited to the production of coal. In point of fact, spore-cases, though often abundantly present, constitute only an infinitesimal part of the matter of the great coal-beds. In an article in "The American Journal of Science," which appeared shortly after that above referred to, I endeavoured to correct this error, though apparently without effect in so far as the majority of British geological writers are concerned. From this article I have taken with little change the following passages, as it is of importance in theoretical geology that such mistakes, involving as they do the whole theory of coal accumulation, should not continue to pass current. The early part of the paper is occupied with facts as to the occurrence of spores and spore-cases as partial ingredients in coal. Its conclusions are as follows: It is not improbable that sporangites, or bodies resembling them, may be found in most coals; but it is most likely that their occurrence is accidental rather than essential to coal accumulation, and that they are more likely to have been abundant in shales and cannel coals, deposited in ponds or in shallow waters in the vicinity of lycopodiaceous forests, than in the swampy or peaty deposits which constitute the ordinary coals. It is to be observed, however, that the conspicuous appearance which these bodies, and also the strips and fragments of epidermal tissue, which resemble them in texture, present in slices of coal, may incline an observer, not having large experience in the examination of coals, to overrate their importance; and this I think has been done by most microscopists, especially those who have confined their attention to slices prepared by the lapidary. One must also bear in mind the danger arising from mistaking concretionary accumulations of bituminous matter for sporangia. In sections of the bituminous shales accompanying the Devonian coal above mentioned, there are many rounded yellow spots, which on examination prove to be the spaces in the epidermis of Psilophyton through which the vessels passing to the leaves were emitted. To these considerations I would add the following, condensed from the paper above referred to (p. 139), in which the whole question of the origin of coal is fully discussed:

"Journal of the Geological Society," vol. xv.

Ibid., vol. xxii.

See also "Acadian Geology," 2d ed., pp. 138, 461, 493.

1. The mineral charcoal or 'mother coal' is obviously woody tissue and fibres of bark, the structure of the varieties of which, and the plants to which it probably belongs, I have discussed in the paper above mentioned.

2. The coarser layers of coal show under the microscope a confused mass of fragments of vegetable matter belonging to various descriptions of plants, and including, but not usually in large quantities, sporangites.

3. The more brilliant layers of the coal are seen, when separated by thin laminæ of clay, to have on their surfaces the markings of Sigillariæ and other trees, of which they evidently represent flattened specimens, or rather the bark of such specimens. Under the microscope, when their structures are preserved, these layers show cortical tissues more abundantly than any others.

4. Some thin layers of coal consist mainly of flattened layers of leaves of Cordaites or Pychnophyllum.

5. The Stigmaria under-clays and the stumps of Sigillaria in the coal-roofs equally testify to the accumulation of coal by the growth of successive forests, more especially of Sigillariæ. There is, on the other hand, no necessary connection of sporangite-beds with Stigmarian soils. Such beds are more likely to be accumulated in water, and consequently to constitute bituminous shales and cannels.

6. Lepidodendron and its allies, to which the spore-cases in question appear to belong, are evidently much less important to coal accumulation than Sigillaria, which cannot be affirmed to have produced spore-cases similar to those in question, even though the observation of Goldenberg as to their fruit can be relied on; the accuracy of which, however, I am inclined to doubt.

On the whole, then, while giving due credit to those who have advocated the spore-theory of coal, for directing attention to this curious and no doubt important constituent of mineral fuel, and admitting that I may possibly have given too little attention to it, I must maintain that sporangite-beds are exceptional among coals, and that cortical and woody matters are the most abundant ingredients in all the ordinary kinds; and to this I cannot think that the coals of England constitute an exception.

It is to be observed, in conclusion, that the spore-cases of plants, in their indestructibility and richly carbonaceous character, only partake of qualities common to most suberous and epidermal matters, as I have explained in the publications already referred to. Such epidermal and cortical substances are extremely rich in carbon and hydrogen, in this resembling bituminous coal. They are also very little liable to decay, and they resist more than other vegetable matters aqueous infiltration--properties which have caused them to remain unchanged, and to continue free from mineral additions more than other vegetable tissues. These qualities are well seen in the bark of our American white birch. It is no wonder that materials of this kind should constitute considerable portions of such vegetable accumulations as the beds of coal, and that when present in large proportion they should afford richly bituminous beds. All this agrees with the fact, apparent on examination of the common coal, that the greater number of its purest layers consist of the flattened bark of Sigillariæ and similar trees, just as any single flattened trunk embedded in shale becomes a layer of pure coal. It also agrees with the fact that other layers of coal, and also the cannels and earthy bitumens, appear under the microscope to consist of finely comminuted particles, principally of epidermal tissues, not only from the fruits and spore-cases of plants, but also from their leaves and stems. These considerations impress us, just as much as the abundance of spore-cases, with the immense amount of the vegetable matter which has perished during the accumulation of coal, in comparison with that which has been preserved.

I am indebted to Dr. T. Sterry Hunt for the following very valuable information, which at once places in a clear and precise light the chemical relations of epidermal tissue and spores with coal. Dr. Hunt says: "The outer bark of the cork-tree, and the cuticle of many if not all other plants, consists of a highly carbonaceous matter, to which the name of suberin has been given. The spores of Lycopodium also approach to this substance in composition, as will be seen by the following, one of two analyses by Duconi, along with which I give the theoretical composition of pure cellulose or woody fibre, according to Payen and Mitscherlich, and an analysis of the suberin of cork, from Quercus suber, from which the ash and 2·5 per cent of cellulose have been deducted.

Liebig and Kopp, "Jahresbuch," 1847-'48.

Gmelin, "Handbook," xv., 145.

---------+-----------+--------+------------ | Cellulose.| Cork. | Lycopodium. ---------+-----------+--------+------------ Carbon | 44·44 | 65·73 | 64·80 Hydrogen | 6·17 | 8·33 | 8·73 Nitrogen | .... | 1·50 | 6·18 Oxygen | 49·39 | 24·44 | 20·29 +-----------+--------+------------ Total | 100·00 | 100·00 | 100·00 ---------+-----------+--------+------------

"This difference is not less striking when we reduce the above centesimal analyses to correspond with the formula of cellulose, C{24}H{20}O{20}, and represent cork and Lycopodium as containing twenty-four equivalents of carbon. For comparison I give the composition of specimens of peat, brown coal, lignite, and bituminous coal:

"Canadian Naturalist," vi., 253.

Cellulose C{24}H{20}O{20}

Cork C{24}H{18-2/10}O{6-7/10}

Lycopodium C{24}H{19-4/10}NO{5-6/10}

Peat (Vaux) C{24}H{14-4/10}O{10}

Brown coal (Schröther) C{24}H{14-3/10}O{10-6/10}

Lignite (Vaux) C{24}H{11-3/10}O{6-4/10}

Bituminous coal (Regnault) C{24}H{10}O{3-3/10}

"It will be seen from this comparison that, in ultimate composition, cork and Lycopodium are nearer to lignite than to woody fibre, and may be converted into coal with far less loss of carbon and hydrogen than the latter. They in fact approach closer in composition to resins and fats than to wood, and, moreover, like those substances repel water, with which they are not easily moistened, and thus are able to resist those atmospheric influences which effect the decay of woody tissue."

I would add to this only one further consideration. The nitrogen present in the Lycopodium spores, no doubt, belongs to the protoplasm contained in them, a substance which would soon perish by decay; and subtracting this, the cell-walls of the spores and the walls of the spore-cases would be most suitable material for the production of bituminous coal. But this suitableness they share with the epidermal tissue of the scales of strobiles, and of the stems and leaves of ferns and lycopods, and, above all, with the thick, corky envelope of the stems of Sigillariæ and similar trees, which, as I have elsewhere shown, from its condition in the prostrate and erect trunks contained in the beds associated with coal, must have been highly carbonaceous and extremely enduring and impermeable to water. In short, if, instead of "spore-cases," we read "epidermal tissues in general, including spore-cases," all that has been affirmed regarding the latter will be strictly and literally true, and in accordance with the chemical composition, microscopical characters, and mode of occurrence of coal. It will also be in accordance with the following statement, from my paper on the "Structures in Coal," published in 1859:

"Vegetable Structures in Coal," "Journal of Geological Society," xv., 626. "Conditions of Accumulation of Coal," ibid., xxii., 95. "Acadian Geology," 197, 464.

"A single trunk of Sigillaria in an erect forest presents an epitome of a coal-seam. Its roots represent the Stigmaria under-clay; its bark the compact coal; its woody axis the mineral charcoal; its fallen leaves (and fruits), with remains of herbaceous plants growing in its shade, mixed with a little earthy matter, the layers of coarse coal. The condition of the durable outer bark of erect trees concurs with the chemical theory of coal, in showing the especial suitableness of this kind of tissue for the production of the purer compact coals. It is also probable that the comparative impermeability of the bark to mineral infiltration is of importance in this respect, enabling this material to remain unaffected by causes which have filled those layers, consisting of herbaceous materials and decayed wood, with pyrites and other mineral substances."

We need not go far in search of the uses of the coal vegetation, when .we consider the fact that the greatest civilised nations are dependent on it for their fuel. Without the coal of the Carboniferous period and the iron-ore which is one of the secondary consequences of coal accumulation, just as bog-ores of iron occur in the subsoils of modern peats, it would have been impossible either to sustain great nations in comfort in the colder climates of the northern hemisphere or to carry on our arts and manufactures. The coal-formation yields to Great Britain alone about one hundred and sixty million tons of coal annually, and the miners of the United States extract mainly from the same formation nearly a hundred million tons, while the British colonies and dependencies produce about five million tons; and it is a remarkable fact that it is to the English race that the greatest supply of this buried power and heat and light has been given.

The great forests of the coal period, while purifying the atmosphere of its excess of unwholesome carbonic acid, were storing up the light and heat of Palæozoic summers in a form in which they could be recovered in our human age, so that, independently of their uses to the animals which were their contemporaries, they are indispensable to the existence of civilised man.

Nor can we hope soon to be able to dispense with the services of this accumulated store of fuel. The forests of to-day are altogether insufficient for the supply of our wants, and though we are beginning to apply water-power to the production of electricity, and though some promising plans have been devised for the utilisation of the direct heat and light of the sun, we are still quite as dependent as any of our predecessors on what has been done for us in the Palæozoic age.

In the previous pages I have said little respecting the physical geography of the Carboniferous age; but, as may be inferred from the vegetation, this in the northern hemisphere presented a greater expanse of swampy flats little elevated above the sea than we find in any other period. As to the southern hemisphere, less is known, but the conditions of vegetation would seem to have been essentially the same.

Taking the southern hemisphere as a whole, I have not seen any evidence of a Lower Devonian or Upper Silurian flora; but in South Africa and Australia there are remains of Upper Devonian or Lower Carboniferous plants. These were succeeded by a remarkable Upper Carboniferous or Permian group, which spread itself all over India, Australia, and South Africa, and contains some forms (Vertebraria, Phyllotheca, Glossopteris, &c. ) not found in rocks of similar age in the northern hemisphere, so that, if the age of these beds has been correctly determined, the southern hemisphere was in advance in relation to some genera of plants. This, however, is to be expected when we consider that the Triassic and Jurassic flora of the north contains or consists of intruders from more southern sites. These beds are succeeded in India by others holding cycads, &c., of Upper Jurassic or Lower Cretaceous types (Rajmahal and Jabalpur groups).

Wyley, "Journal Geol. Society," vol. xxiii., p. 172; Daintree, ibid., vol. xxviii.; also Clarke and McCoy.

Blanford has shown that there is a very great similarity in this series all over the Australian and Indian region. Hartt and Darby have in like manner distinguished Devonian and Carboniferous forms in Brazil akin to those of the northern hemisphere. Thus the southern hemisphere would seem to have kept pace with the northern, and according to Blanford there is evidence there of cold conditions in the Permian, separating the Palæozoic flora from that of the Mesozoic, in the same manner that Ramsay has supposed a similar period of cold to have done north of the equator. This would imply a very great change of climate, since we have evidence of the extension of the Lower Carboniferous flora at least as far north as Spitzbergen. The upper coal-formation we cannot, however, trace nearly so far north; so that a gradual refrigeration may have been going on before the Permian. Thus in both hemispheres there was a general similarity in the later Palæozoic flora, and perhaps similar conditions leading to its extinction and to its replacement by that to be described in the next chapter.

"Journal Geol. Society," vol. xxxi.

* * * * *

NOTES TO CHAPTER IV.

I. Characters and Classification of Palæozoic Plants.

In the space available in this work it would be impossible to enter fully into the classification of Palæozoic plants; but it may be well to notice some important points for the guidance of those who may desire to collect specimens; more especially as much uncertainty exists as to affinities and very contradictory statements are made. The statements below may be regarded as the results of actual observation and of the study of specimens in situ in the rocks, as well as in the cabinet and under the microscope.

Gymnospermeæ.

Family Coniferæ; Genus Dadoxylon, Endlicher; Araucarites, Goeppert; Araucarioxylon, Kraus.

The trunks of this genus occur from the Middle Devonian to the Permian inclusive, as drift-logs calcified, silicified, or pyritised. The only foliage associated with them is of the type of Walchia and Araucarites--viz., slender branches with numerous small spiral acicular leaves. Two of the coal-formation species, D. materiarum and another, had foliage of this type. That of the others is unknown. They are all distinct from the wood of Cordaites, for which see under that genus.

The following are North American species:

Trunks.

Dadoxylon Ouangondianum, Dn. M. Erian Report, 1871. D. Halli, Dn. " " D. Newberryi, Dn. " " D. Clarkii, Dn. (Cordæoxylon ?) " Report, 1882. D. Acadianum, Dn. Coal-formation Acadian Geology. and millstone grit. D. Materiarum, Dn. Do. and Permo- " Carb. D. (Palæoxylon) antiquius, Dn. L. Carboniferous. " D. annulatum, Dn. Coal-formation. " Ormoxylon Erianum, Dn. Erian Report, 1871.

Foliage.

Araucarites gracilis, Dn. N. Coal-formation " and Permian. { Report on Walchia robusta, Dn. Permian. { Prince Edward W. imbricatula, Dn. " { Island.

"Geological Survey of Canada: Fossil Plants of Erian and Upper Silurian Formations," by J. W. Dawson.

All of the above can be vouched for as good species based upon microscopic examination of a very large number of trunks from different parts of North America. The three Erian species of Dadoxylon and D. antiquius from the Lower Carboniferous have two or more rows of cells in the medullary rays. The last named has several rows, and is a true Palæoxylon allied to D. Withami of Great Britain. D. materiarium is specially characteristic of the upper coal-formation and Permian, and to it must belong one or both of the species of foliage indicated above. D. Clarkii has very short, simple medullary rays of only a few cells superimposed, and has an inner cylinder of scalariform vessels, approaching in these points to Cordaites. Ormoxylon has a very peculiar articulated pith and simple medullary rays.

Witham in 1833 described several Carboniferous species of pine-wood, under the generic name Pinites, separating under the name Pitus species which appeared to have the discs on the cell-walls separate and in transverse lines. Witham's name was changed by Goeppert to Araucarites, to indicate the similarity of these woods to Araucaria, Pinites being reserved for trees more closely allied to the ordinary pines. Endlicher, restricting Araucarites to foliage, etc., of Araucaria-like trees, gave the name Dadoxylon to the wood; and this, through Unger's "Genera and Species," has gained somewhat general acceptance. Endlicher also gave the name Pissadendron to the species which Witham had called Pitus; but Brongniart proposed the name Palæoxylon to include all the species with thick and complex medullary rays, whatever the arrangement of the discs. In Schimper's new work Kraus substitutes Araucarioxylon for Endlicher's Dadoxylon, and includes under Pissadendron all the species placed by Brongniart in Palæoxylon.

To understand all this confusion, it may be observed that the characters available in the determination of Palæozoic coniferous wood are chiefly the form and arrangement of the wood-cells, the character of the bordered pores or discs of their walls, and the form and composition of the medullary rays.

The character on which Witham separated his genus Pitus from Pinites is, as I have ascertained by examination of slices of one of his original specimens kindly presented to me by Mr. Sanderson, of Edinburgh, dependent on state of preservation, the imperfectly preserved discs or areolations of the walls of the fibre presenting the appearance of separate and distinct circles, while in other parts of the same specimens these discs are seen to be contiguous and to assume hexagonal forms, so that in this respect they do not really differ from the ordinary species of Dadoxylon. The true character for subdividing those species which are especially characteristic of the Carboniferous, is the composite structure of the medullary rays, which are thick and composed of several radial piles of cells placed side by side. This was the character employed by Brongniart in separating the genus Palæoxylon, though he might with convenience have retained Witham's name, merely transferring to the genus the species of Witham's Pinites which have complex medullary rays. The Erian rocks present the greatest variety of types, and Palæoxylon is especially characteristic of the Lower Carboniferous, while species of Dadoxylon with two rows of bordered pores and simple medullary rays are especially plentiful in the upper coal-formation and Permo-Carboniferous.

The following table will clearly show the distinctive characters and relations of the genera in question, as held by the several authors above referred to:

Wood of Palæozoic Conifers.

Woody | Medullary rays | Generic | Geological fibres. | and pith. | names. | age. -------------+--------------------+---------------------+--------------- No discs. | One or two series | Aporoxylon, | Devonian | of cells. | Unger. | (Erian). -------------+--------------------+---------------------+--------------- | Complex, or of two | { Pitus, | | or more series of | { Witham. | Middle and | or more series of | { Palæoxylon, | Lower | cells. | { Brongniart. | Carboniferous | Pith Sternbergian. | { Pissadendron, | and Devonian. | | { Endlicher. | +--------------------+---------------------+--------------- Discs in one | Simple, or of one | { Araucarites, | series | row of cells. | { Goeppert | Upper contiguous, | Pith Sternbergian. | { Dadoxylon, | Carboniferous or in | | { Endlicher. | and Permian. several | | { Araucarioxylon, | series | | { Schimper. | spirally +--------------------+---------------------+--------------- arranged. | Pith in spherical | Ormoxylon, Dn. | Devonian. | chambers. | | +--------------------+---------------------+--------------- | Medullary sheath | | Devonian. | scalariform. | Dadoxylon | | Medullary rays | (Cordæoxylon), | | frequent, simple, | Dn. | | short. | | -------------+--------------------+---------------------+---------------

Type O. Erianum, Dn., "Report on Canadian Plants," 1871.

Type D. Clarkii, Dn., "Report on Canadian Plants," 1882. This may be wood of Cordaites, to which it approaches very closely.

Family Cordaites, Genus Cordaites, Brongniart.

Trunks marked by transverse scars of attachment of bases of leaves; leaves broad, with many parallel veins, and attached by a broad base; pistillate and staminate catkins of the nature of Antholithes. Fruit winged or pulpy, of the kind known as Cardiocarpum. Stem with a Sternbergia pith, usually large, surrounded by a ring of pseudo-scalariform vessels, and with a cylinder usually narrow, of woody wedges, with bordered pores in one or more series, and with simple medullary rays.

From specimens kindly presented to me by Prof. Renault, I have been able to ascertain that the stems of some at least of these plants (Eucordaites) are distinct in structure from all the species of Dadoxylon, above mentioned, except D. Clarkii, of the Erian. They may be regarded as intermediate between those of conifers and cycads, which is indeed the probable position of these remarkable plants.

Grand d'Eury has divided the Cordaites into sub-genera, as follows:

1. Eucordaites.--Leaves spatulate, obovate, elliptical, or lanceolate, sessile, entire, with rounded apices and of leathery consistency. The leaves are from twenty to ninety centimetres in length. The nerves are either equally or unequally strong.

2. Dorycordaites.--Leaves lanceolate, with sharp points; nerves numerous, fine, and equal in strength. The leaves attain a length of from forty to fifty centimetres.

3. Poacordaites.--Leaves narrow, linear, entire, blunt at the point, with nerves nearly equally strong. The leaves are as much as forty centimetres in length.

To these Renault and Zeiller have added a fourth group, Scutocordaites.

Genus Sternbergia.

This is merely a provisional genus intended to receive casts of the pith cylinders of various fossil trees. Their special peculiarity is that, as in the modern Cecropia peltata, and some species of Ficus, the pith consists of transverse dense partitions which, on the elongation of the internodes, become separated from each other, so as to produce a chambered pith cavity, the cast of which shows transverse furrows. The young twigs of the modern Abies balsamifera present a similar structure on a minute scale. I have ascertained and described such pith-cylinders in large stems of Dadoxylon Ouangondianum, and D. materiarium. They occur also in the stems of Cordaites and probably of Sigillariæ. I have discussed these curious fossils at length in "Acadian Geology" and in the "Journal of the Geological Society of London," 1860. The following summary is from the last-mentioned paper:

a. As Prof. Williamson and the writer have shown, many of the Sternbergia piths belong to coniferous trees of the genus Dadoxylon.

b. A few specimens present multiporous tissue, of the type of Dictyoxylon, a plant of unknown affinities, and which, according to Williamson, has a Sternbergia pith.

c. Other examples show a true scalariform tissue, comparable with that of Lepidodendron or Sigillaria, but of finer texture. Corda has shown that plants of the type of the former genus (his Lomatophloios) had Sternbergia piths. Some plants of this group are by external characters loosely reckoned by botanists as ribless Sigillariæ (Clathraria); but I believe that they are not related even ordinally to that genus.

d. Many Carboniferous Sternbergiæ show structures identical with those described above as occurring in Cordaites, and also in some of the trees ordinarily reckoned as Sigillariæ.

Genus Cardiocarpum.

I have found at least eight species of these fruits in the Erian and Carboniferous of New Brunswick and Nova Scotia, all of which are evidently fruits of gymnospermous trees. They agree in having a dense coaly nucleus of appreciable thickness, even in the flattened specimens, and surrounded by a thin and veinless wing or margin. They have thus precisely the appearance of samaras of many existing forest-trees, some of which they also resemble in the outline of the margin, except that the wings of samaras are usually veiny. The character of the nucleus, and the occasional appearance in it of marks possibly representing cotyledons or embryos, forbids the supposition that they are spore-cases. They must have been fruits of phænogams. Whether they were winged fruits or seeds, or fruits with a pulpy envelope like those of cycads and some conifers, may be considered less certain. The not infrequent distortion of the margin is an argument in favour of the latter view, though this may also be supposed to have occurred in samaras partially decayed. On the other hand, their being always apparently flattened in one plane, and the nucleus being seldom, if ever, found denuded of its margin, are arguments in favour of their having been winged nutlets or seeds. Until recently I had regarded the latter view as more probable, and so stated the matter in the second edition of "Acadian Geology." I have, however, lately arrived at the conclusion that the Cardiocarpa of the type of C. cornutum were gymnospermous seeds, having two cotyledons embedded in an albumen and covered with a strong membranous or woody tegmen surrounded by a fleshy outer coat, and that the notch at the apex represents the foramen or micropyle of the ovule. The structure was indeed very similar to that of the seeds of Taxus and of Salisburia. With respect to some of the other species, however, especially those with very broad margins, it still appears likely that they were winged.

The Cardiocarpa were borne in racemes or groups, and it seems certain that some of them at least are the seeds of Cordaites. The association of some of them and of those of the next genus with Sigillariæ is so constant that I cannot doubt that some of them belong to plants of that genus, or possibly to taxine conifers. The great number of distinct species of these seeds, as compared with that of known trees which could have produced them, is very remarkable.

Genus Trigonocarpum.

These are large angled nuts contained in a thick envelope, and showing internal structures resembling those of the seeds of modern Taxineæ. There are numerous species, as well as allied seeds referred to the provisional genera Rhabdocarpus and Carpolithes. In Trigonocarpum Hookeri I have described the internal structure of one of those seeds, and many fine examples from the coal-field of St. Etienne, in France, have been described by Brongniart, so that their internal structure is very well known.

Genus Antholithes.

This is also a provisional genus, to include spikes of floral organs, some of which are known to have belonged to Cordaites, others probably to Sigillariæ.

Of Uncertain Affinities.

Family Sigillariaceæ.

Under this name palæobotanists have included a great number of trees of the Carboniferous system, all of which are characterised by broad leaf-sears, with three vascular scars, and usually arranged in vertical rows, and by elongated three-nerved leaves, and roots of the stigmaria type--that is, with rounded pits, marking the attachment of rootlets spirally arranged. These trees, however, collected in the genus Sigillaria by arbitrary characters, which pass into those of the Lepidodendroid trees, have been involved in almost inextricable confusion, to disentangle which it will be necessary to consider: 1. The external characters of Sigillariæ, and trees confounded with them. 2. Subdivision of Sigillariæ by external markings. 3. The microscopic character of their stems. 4. What is known of their foliage and fruit.

1. Characters of Sigillaroid and Lepidodendroid Trunks.

It may be premised that the modes of determination in fossil botany are necessarily different from those employed in recent botany. The palæobotanist must have recourse to characters derived from the leaves, the scars left by their fall, and the internal structures of the stem. These parts, held in little esteem by botanists in describing modern plants, and much neglected by them, must hold the first place in the regard of the fossil botanist, whereas the fructification, seldom preserved, and generally obscure, is of comparatively little service. It is to be remarked also that in such generalised plants as those of the Palæozoic, remarkable rather for the development of the vegetative than of the reproductive organs, the former rise in importance as compared with their value in the study of modern plants.

In Sigillariæ, Lepidodendra, &c., the following surfaces of the stem may be presented to our inspection:

1. The outer surface of the epidermis without its leaves, but with the leaf-bases and leaf-scars more or less perfectly preserved. On this surface we may recognise: (1) Cellular swellings or projections of the bark to which the leaves are attached. These may be called leaf-bases, and they are sometimes very prominent. (2) The actual mark of the attachment of the leaf situated in the most prominent part of the leaf-base. This is the leaf-scar. (3) In the leaf-scar when well preserved we can see one or more minute punctures or prominences which are the points where the vascular bundles passing to the leaf found exit. These are the vascular scars.

When the leaves are attached, the leaf-scars and vascular scars cannot be seen, but the leaf-bases can be made out. Hence it is important, if possible, to secure specimens with and without the leaves. In flattened specimens the leaf-bases are often distorted by pressure and marked with furrows which must not be mistaken for true structural characters. The leaf-bases, which are in relief on the outer surface of the stem, of course appear as depressions on the mould in the containing rock, in which the markings often appear much more distinctly than on the plant itself.

2. The outer surface of the epidermis may have been removed or may be destroyed by the coarseness of the containing rock. In this case the leaf-bases are usually preserved on the surface of the outer or corky bark, but the leaf-scars and vascular scars have disappeared. This gives that condition of Lepidodendroid trees to which the name Knorria has been applied. When plants are in this state careful inspection may sometimes discover traces of the leaf-scars on portions of the stem, and thus enable the Knorria to be connected with the species to which it belongs.

3. The outer or corky bark may be removed, exposing the surface of the inner or fibrous and cellular bark, which in the plants in question is usually of great thickness. In this case neither the leaf-bases nor the scars are seen, but punctures or little furrows or ridges appear where the vascular bundles entered the inner bark. Specimens in this state are usually said to be decorticated, though only the outer bark is removed. It is often difficult to determine plants in this condition, unless some portion of the stem can be found still retaining the bark; but when care is taken in collecting, it will not infrequently be found that the true outer surface can be recovered from the containing rock, especially if a coaly layer representing the outer bark intervenes between this and the inner impression. Specimens of this kind, taken alone, have been referred to the genera Knorria, Bothrodendron, and Halonia.

4. In some cases, though not frequently, the outer surface of the ligneous cylinder is preserved. It almost invariably presents a regularly striated or irregularly wrinkled appearance, depending upon the vertical woody wedges, or the positions of the medullary rays or vascular bundles. Specimens of this kind constituted some of the Endogenites of the older botanists, and the genus Schizodendron of Eichwald appears to include some of them. Many of them have also been incorrectly referred to Calamites.

5. In some cases the cast of the medullary cylinder or pith may alone be preserved. This may be nearly smooch or slightly marked by vertical striæ, but more usually presents a transverse striation, and not infrequently the transverse constrictions and septa characteristic of the genus Sternbergia. Loose Sternbergiæ afford little means of connecting them with the species to which they belong, except by the microscopic examination of the shreds of the ligneous cylinder which often cling to them.

See my paper, "Journal of Geological Society," vol. xxvii.

These facts being premised, the following general statements may be made respecting some of the more common Palæozoic genera, referring, however, principally to the perfect markings as seen on the epidermis:

Sigillaria.--Leaf-bases hexagonal or elongated, or confluent on a vertical ridge. Leaf-scars hexagonal or shield-shaped. Vascular scars three, the two lateral larger than the central. This last character is constant, depending on the fact that the leaves of Sigillaria have two or more vascular bundles. All so-called Sigillariæ having the central vascular scar largest, or only one vascular bundle, should be rejected from this genus. In young branches of branching Sigillariæ the leaf-scars sometimes appear to be spiral, but in the older stems they form vertical rows; interrupted, however, by transverse rows or bands of fruit-scars, each with a single large central vascular scar, and which have borne the organs of fructification. Arthrocaulis of McCoy is founded on this peculiarity.

Syringodendron.--Differs from Sigillaria in the leaf-scars, which are circular and with a single vascular bundle. It is a matter of doubt whether these plants were of higher rank than Sigillaria tending toward the pines, or of lower rank tending toward Cyclostigma. Their leaf-bases form vertical ridges.

Lepidodendron.--Leaf-bases rhombic, oval, or lanceolate, moderately prominent. Leaf-scars rhombic or sometimes shield-shaped or heart-shaped, in the middle or upper part of the leaf-base. Vascular scars three--the middle one always largest and corresponding to the single nerve of the leaf; the lateral ones sometimes obsolete.

In older stems three modes of growth are observed. In some species the expansion of the bark obliterates the leaf-bases and causes the leaf-scars to appear separated by wide spaces of more or less wrinkled bark, which at length becomes longitudinally furrowed and simulates the ribbed character of Sigillaria. In others the leaf-bases grow in size as the trunk expands, so that even in large trunks they are contiguous though much larger than those on the branches. In others the outer bark, hardening at an early age, is incapable of either of the above changes, and merely becomes cleft into deep furrows in the old trunks.

Lepidophloios.--Leaf-bases transverse and prominent--often very much so. Leaf-scars transversely rhombic or oval with three vascular scars, the central largest. Leaves very long and one-nerved. Large strobiles or branchlets borne in two ranks or spirally on the sides of the stem, and leaving large, round scars (cone-scars), often with radiating impressions of the basal row of scales.

Species with long or drooping leaf-bases have been included in Lepidophloios and Lomatophloios, Species with short leaf-bases and cone-scars in two rows have been called Ulodendron, and some of them have been included in Sigillaria (sub-genus Clathraria). Decorticated stems are Bothrodendron and Halonia. Some of the species approach near to the last genus, especially to the Lepidodendra with rhombic leaf-bases like L. tetragonum.

Cyclostigma.--Leaf-bases undeveloped. Leaf-scars circular or horseshoe-shaped, small, with a central vascular scar. In old trunks of Cyclostigma the leaf-scars become widely separated, and sometimes appear in vertical rows. Young branches of Lepidodendron sometimes have the leaf-scars similar to those of Cyclostigma.

Leptophleum.--Leaf-bases flat, rhombic; leaf-scars obsolete; vascular scar single, central. The last two genera are characteristically Devonian.

In contradistinction from the trees above mentioned, the following general statements may be made respecting other groups:

In conifers the leaf-bases are usually elongated vertically, often scaly in appearance, and with the leaf-scar terminal and round, oval, or rhombic, and with a single well-marked vascular scar.

In Calamites, Calamodendron, and Asterophyllites the scars of the branchlets or leaves are circular or oval, with only a single vascular scar, and situated in verticils at the top of well-marked nodes of the stem.

In tree-ferns the leaf-bases are large and usually without a distinct articulating surface. The vascular bundles are numerous. Protopteris has rounded leaf-scars with a large horseshoe-shaped bundle of vessels above and small bundles below. Caulopteris has large elliptic or oval leaf-scars with vascular scars disposed concentrically. Palæopteris, of Geinitz, has the leaf-scars transversely oval and the vascular bundles confluent in a transverse band with an appendage or outlying bundle below. Stemmatopteris has leaf-scars similar to those of Caulopteris, but the vascular bundles united into a horseshoe-shaped band.

This name, preoccupied by Geinitz, has been inadvertently misapplied to the Devonian ferns of the genus Archæopteris.

2. Subdivision of Sigillariæ in Accordance with their Markings.

The following groups may be defined in this way; but, being based on one character only, they are of course in all probability far from natural:

1. Sigillaria, Brongniart. Type, Sigillaria reniformis, Brongniart, or S. Brounii, Dawson.--Stem with broad ribs, usually much broader than the usually oval or elliptical tripunctate areoles, but disappearing at base, owing to expansion of the stem. Leaves narrow, long, three-nerved.

2. Rhytidolepis, Sternberg. Type, S. scutellata, Brongniart.--Ribs narrow, and often transversely striate. Areoles large, hexagonal or shield-shaped, tripunctate. Leaves as in last group. Kings of rounded scars on the stems and branches mark attachment of fruit. It is possible that some of the smaller stems of this group may be branches of trees of group first.

3. Syringodendron, Sternberg. Type, S. organum, L. and H., S. oculata, Brongniart.--Stems ribbed; areoles small and round, and apparently with a single scar, or three closely approximated. These are rare, and liable to be confounded with decorticated examples of other groups; but I have some specimens which unquestionably represent the external surface.

4. Favularia, Sternberg. Type, Sigillaria elegans of Brongniart.--Leaf-bases hexagonal, or in young branches elliptical, in vertical rows, but without distinct ribs, except in old or decorticated stems. Fruit borne in verticils on the branches bearing transverse rows of rounded scars. Leaves somewhat broad and longitudinally striate.

5. Leioderma, Goldenberg. Type, S. Sydnensis, Dawson.--Ribs obsolete. Cortical and ligneous surfaces striate. Vascular scars double, elongate longitudinally, and alike on cortical and inner surfaces. Areoles in rows and distinct; stigmaria-roots striate, with small and distinct areoles.

6. Clathraria, Brongniart. Type, S. Menardi, Brongniart.--Areoles hexagonal, not in distinct rows, but having a spiral appearance. Some of the plants usually referred to this group are probably branches of Favularia. Others are evidently fragments of plants of the genus Lepidophloios.

3. Internal Structures of Sigillaria-Stems.

I long ago pointed out, on the evidence of the external markings and mode of growth, that the stems of Sigillariæ must have been exogenous, and this conclusion has now been fully confirmed by the microscopic researches of Williamson, not only in the case of Sigillariæ, but of Lepidodendra and Calamodendra as well. Confining myself to my own observations, three types of Sigillariæ are known to me by their internal structures, though I cannot certainly correlate all of these with the external markings referred to above.

1. Diploxylon, in which the stem consists of a small internal axis surrounded by a very thick inner bark and a dense outer cortex. A fine example from the South Joggins is thus described:

"Journal of the Geological Society of London," November, 1877.

"The axis of the stem is about six centimetres in its greatest diameter, and consists of a central pith-cylinder and two concentric coats of scalariform tissue. The pith-cylinder is replaced by sandstone, and is about one centimetre in diameter. The inner cylinder of scalariform tissue is perfectly continuous, not radiated, and about one millimetre in thickness. Its vessels are somewhat crushed, but have been of large diameter. Its outer surface, which readily separates from that of the outer cylinder, is striated longitudinally. The outer cylinder, which constitutes by much the largest part of the whole, is also composed of scalariform tissue; but this is radially arranged, with the individual cells quadrangular in cross-section. The cross-bars are similar on all the sides and usually simple and straight, but sometimes branching or slightly reticulated. The wall intervening between the bars has extremely delicate longitudinal waving lines of ligneous lining, in the manner first described by Williamson as occurring in the scalariform tissue of certain Lepidodendra. A few small radiating spaces, partially occupied with pyrites, obscurely represent the medullary rays, which must have been very feebly developed. The radiating bundles passing to the leaves run nearly horizontally; but their structure is very imperfectly preserved. The stem being old and probably long deprived of its leaves, they may have been partially disorganised before it was fossilised. The outer surface of the axis is striated longitudinally, and in some places marked with impressions of tortuous fibres, apparently those of the inner bark. In the cross-section, where weathered, it shows concentric rings; but under the microscope these appear rather as bands of compressed tissue than as proper lines of growth. They are about twenty in number. This tree has an erect, ribbed trunk, twelve feet in height and fifteen inches in diameter, swelling to about two feet at the base."

2. Favularia Type.--This has been well described by Brongniart and by Renault, and differs from the above chiefly in the fact that the outer exogenous woody zone is composed of reticulated instead of scalariform tissue, and the inner zone is of the peculiar form which I have characterised as pseudo-scalariform.

"Botanique Fossile," Paris, 1881.

3. Sigillaria Proper.--This I have illustrated in my paper in the "Journal of the Geological Society" for May, 1871, and it appears to represent the highest and most perfect type of the larger ribbed Sigillaria. This structure I have described as follows, basing my description on a very fine axis found in an erect stem, and on the fragments of the woody axis found in the bases of other erect stems:

a. A dense cellular outer bark, usually in the state of compact coal--but when its structure is preserved, showing a tissue of thickened parenchymatous cells.

b. A very thick inner bark, which has usually in great part perished, or been converted into coal, but which, in old trunks, contained a large quantity of prosenchymatous tissue, very tough and of great durability. This "bast-tissue" is comparable with that of the inner bark of modern conifers, and constitutes much of the mineral charcoal of the coal-seams.

c. An outer ligneous cylinder, composed of wood-cells, either with a single row of large bordered pores, in the manner of pines and cycads, or with two, three, or four rows of such pores sometimes inscribed in hexagonal areoles in the manner of Dadoxylon. This woody cylinder is traversed by medullary rays, which are short, and composed of few rows of cells superimposed. It is also traversed by oblique radiating bundles of pseudo-scalariform tissue proceeding to the leaves. In some Sigillariæ this outer cylinder was itself in part composed of pseudo-scalariform tissue, as in Brongniart's specimen of S. elegans; and in others its place may have been taken by multiporous tissue, as in a case above referred to; but I have no reason to believe that either of these variations occurred in the typical ribbed species now in question. The woody fibres of the outer cylinder may be distinguished most readily from those of conifers, as already mentioned, by the thinness of their walls, and the more irregular distribution of the pores. Additional characters are furnished by the medullary rays and the radiating bundles of scalariform tissue when these can be observed.

These are the same with the wood-cells elsewhere called discigerous tissue, and to which I have applied the terms uniporous and multiporous. The markings on the walls are caused by an unlined portion of the cell-wall placed in a disk or depression, and this often surrounded by an hexagonal rim of thickened wall; but in all cases these structures are less pronounced than in Dadoxylon, and less regular in the walls of the same cell, as well as in different layers of the tissues of the axis.

d. An inner cylinder of pseudo-scalariform tissue. I have adopted the term pseudo-scalariform for this tissue, from the conviction that it is not homologous with the scalariform ducts of ferns and other acrogens, but that it is merely a modification of the discigerous wood-cells, with pores elongated transversely, and sometimes separated by thickened bars, corresponding to the hexagonal areolation of the ordinary wood-cells. A similar tissue exists in cycads, and is a substitute for the spiral vessels existing in ordinary exogens.

e. A large medulla, or pith, consisting of a hollow cylinder of cellular tissue, from which proceed numerous thin diaphragms towards the centre of the stem.

These structures of the highest type of Sigillaria are on the one hand scarcely advanced beyond those of Calamopitus, as described by Williamson, and on the other approach to those of Cordaites, as seen in specimens presented to me by Renault.

Finally, as to the fruit of Sigillariæ, I have no new facts to offer. The strobiles or spikes associated with these trees have been variously described as gymnospermous (Renault) or cryptogamous (Groldenberg and Williamson). 1 have never seen them in place. Two considerations, however, have always weighed with me in reference to this subject. One is the constant abundance of Trigonocarpa and Cardiocarpa in the soil of the Sigiliaria forests, as I have studied this at the South Joggins. The other is that the rings of fruit-scars on the branches of Sigiliaria are homologous with leaf-scars, not with branches, and therefore should have borne single carpels and not cones or spikes of inflorescence. These are merely suggestions, but I have no doubt they will be vindicated by future discoveries, which will, I have no doubt, show that in the family Sigillariaceæ we have really two families, one possibly of gymnospermous rank, or at least approaching to this, the other allied to the Lepidodendra.

Cryptogamia.

(Acrogenes.)

Family Lepidodendreæ; Genus Lepidodendron, Sternberg.

These are arboreal Lycopods having linear one-nerved leaves, stems branching dichotomously, and with ovate or rhombic leaf-bases bearing rhombic leaf-scars, often very prominent. The fruit is in scaly strobiles, terminal or lateral, and there are usually, if not always, macrospores and microspores in each strobile. The young branches and stems have a central pith, a cylinder of scalariform tubes sending out ascending bundles to the leaves through a thick cellular and fibrous inner bark, and externally a dense cortex confluent with or consisting of the leaf-bases. Older stems have a second or outer layer of scalariform fibres in wedges with medullary rays, and strengthening the stem by a true exogenous growth, much as in the Diploxylon type of Sigiliaria. The development of this exogenous cylinder is different in amount and rate in different species. This different development of the exogenous axis is accompanied with appropriate external appearances in the stems, and the changes which take place in their markings. These are of three kinds. In some species the areoles, at first close together, become, in the process of the expansion of the stem, separated by intervening spaces of bark in a perfectly regular manner; so that in old stems, while widely separated, they still retain their arrangement, while in young stems they are quite close to one another. This is the case in L. corrugatum. In other species the leaf-scars or bases increase in size in the old stems, still retaining their forms and their contiguity to each other. This is the case in L. undulatum, and generally in those Lepidodendra which have large leaf-bases. In these species the continued vitality of the bark is shown by the occasional production of lateral strobiles on large branches, in the manner of the modern red pine of America. In other species the areoles neither increase in size nor become regularly separated by growth of the intervening bark; but in old stems the bark splits into deep furrows, between which may be seen portions of bark still retaining the areoles in their original dimensions and arrangement. This is the case with L. Pictoense. This cracking of the bark no doubt occurs in very old trunks of the first two types, but not at all to the same extent.

See "Memoirs of Dr. Williamson," in "Philosophical Transactions," for ample details.

As a type of Lepidodendron, I may describe one of the oldest Carboniferous species characteristic of the Lower Carboniferous in America, and corresponding to L. Veltheimianum of Europe.

Lepidodendron Corrugatum, Dawson.--(See Fig. 43, supra.) "Quarterly Journal of Geological Society," vol. xv.; "Acadian Geology," page 451.

Habit of Growth.--Somewhat slender, with long branches and long, slender leaves having a tendency to become horizontal or drooping.

Markings of Stem.--Leaf-bases disposed in quincunx or spirally, elongate, ovate, acute at both ends, but more acute and slightly oblique at the lower end; most prominent in the upper third, and with a slight vertical ridge. Leaf-scars small, rounded, and showing only a single punctiform vascular scar. The leaf-scar on the outer surface is in the upper third of the base; but the obliquity of the vascular bundle causes it to be nearly central on the inside of the epidermis. In young succulent shoots the leaf-scars are contiguous and round as in Cyclostigma, without distinct leaf-bases. In this state it closely resembles L. Olivieri, Eichwald.

Lethæa Rossica, Plate Y, Figs. 12, 13.

In the ordinary young branches the leaf-scars are contiguous, and closely resemble those of L. elegans, Brongt. (Fig. 43 C). As the branches increase in diameter the leaf-scars slightly enlarge and sometimes assume a verticillate appearance (Fig. 43 D). As they still further enlarge they become separated by gradually increasing spaces of bark, marked with many waving striæ or wrinkles (Fig. 43 I, N). At the base of old stems the bark assumes a generally wrinkled appearance without distinct scars.

Knorria or Decorticated States.--Of these there is a great variety, depending on the state of preservation, and the particular longitudinal ridges. Fig. 43 D shows a form in which the vascular bundles appear as cylindrical truncate projections. Other forms show the leaf-bases prominent, or have an appearance of longitudinal ribbing produced by the expansion of the bark.

Structure of Stem.--This is not perfectly preserved in any of my specimens, but one flattened specimen shows a central medulla with a narrow ring of scalariform vessels surrounding it, and constituting the woody axis. The structure is thus similar to that of L. Harcourtii, which I regard as probably the same with the closely allied European species L. Veltheimianum.

Leaves.--These are narrow, one-nerved, curving somewhat rapidly outward (Figs. 43, B, C, D). They vary from one to two inches in length.

Roots.--I have not seen these actually attached, but they occur very abundantly in the under-clays of some erect forests of these plants at Horton Bluff, and are of the character of Stigmariæ (Figs. 30, 31). In some of the under-clays the long, flattened rootlets are excessively abundant, and show the mark of a central vascular bundle.

Fructification.--Cones terminal, short, with many small, acute imbricate scales. Spore-cases globular, smooth (Fig. 43 C). On the surface of some shales and sandstones at Horton there are innumerable round spore-cases of this tree about the size of mustard-seed (Fig. 43 F). Large slabs are sometimes covered with these, and thin layers of shale are filled with flattened specimens.

This is the characteristic species of the Lower Carboniferous coal-measures, occurring in great profusion at Horton Bluff and its vicinity, also at Sneid's Mills near Windsor, Noel and Five-Mile River, at Norton Creek and elsewhere in New Brunswick (Matthew's collection), and at Antigonish (Honeyman's collection).

I have received from the lowest Carboniferous beds of Ohio specimens of this species. According to Rogers and Lesquereux similar forms occur in the Vespertine of Pennsylvania and in the Lower Carboniferous of Illinois. L. Veltheimianum of western Europe and L. glincanum of Russia are closely allied Lower Carboniferous species.

"Journal of Geological Society," November, 1862, p. 313.

For comparisons of these see "Report on Plants of Lower Carboniferous of Canada," p. 21.

A very different type is furnished by a new species from the middle coal-formation of Clifton, New Brunswick.

Lepidodendron Cliftonense, Dawson.--Habit of Growth.--Robust, with thick branches, and leaves several inches in length. Terminal branches becoming slender, with shorter leaves.

Markings of Stem.--Leaf-bases long oval, pointed at ends, enlarging with growth of stem. Leaf-scars central, rhombic, transverse.

Leaves.--One-nerved, acutely pointed, from four inches in length on the larger branches to one inch or less on the branchlets.

Fructification.--Cones large, cylindrical or long oval, with large scales of trigonal form, and not elongated but lying close to the surface. Borne on lateral, slender branchlets, with short leaves.

Genus Lepidophloios, Sternberg; Ulodendron, L. and H.; Lomatophloios, Corda.

Lepidophloios.--Under this generic name, established by Sternberg, I include those lycopodiaceous trees of the coal-measures which have thick branches, transversely elongated leaf-scars, each with three vascular points and placed on elevated or scale-like protuberances, long one-nerved leaves, and large lateral strobiles in vertical rows or spirally disposed. Their structure resembles that of Lepidodendron, consisting of a Sternbergia pith, a slender axis of large scalariform vessels, giving off from its surface bundles of smaller vessels to the leaves, a very thick cellular bark, and a thin dense outer bark, having some elongated cells or bast-tissue on its inner side. In these trees the exogenous outer cylinder is less developed than in the Lepidodendra, and is sometimes wanting in stems or branches of some thickness.

Regarding L. laricinum of Sternberg as the type of the genus, and taking in connection with this the species described by Goldenberg, and my own observations on numerous specimens found in Nova Scotia, 1 have no doubt that Lomatophloios crassicaulis of Corda, and other species of that genus described by Goldenberg, Ulodendron and Bothrodendron of Lindley, Lepidodendron ornatissimum of Brongniart, and Halonia punctata of Geinitz, all belong to this genus, and differ from each other only in conditions of growth and preservation. Several of the species of Lepidostrobus and Lepidophyllum also belong to Lepidophloios.

The species of Lepidophloios are readily distinguished from Lepidodendron by the form of the areoles, and by the round scars on the stem, which usually mark the insertion of the large strobiles, though in barren stems they may also have produced branches; still, the fact of my finding the strobiles in situ in one instance, the accurate resemblance which the scars bear to those left by the cones of the red pine when borne on thick branches, and the actual impressions of the radiating scales in some specimens, leave no doubt in my mind that they are usually the marks of cones; and the great size of the cones of Lepidophloios accords with this conclusion.

The species of Lepidophloios are numerous, and individuals are quite abundant in the coal formation, especially toward its upper part. Their flattened bark is frequent in the coal-beds and their roofs, affording a thin layer of pure coal, which sometimes shows the peculiar laminated or scaly character of the bark when other characters are almost entirely obliterated. The leaves also are nearly as abundant as those of Sigillaria in the coal-shales. They can readily be distinguished by their strong, angular midrib.

The markings of Lepidophloios may easily be mistaken for those of the Clathraria type of Sigillaria. When the stem only is seen, they can be distinguished by the length of the leaf-bases in Lepidophloios, and by the dominant central vascular scar; also by the one-nerved and ribbed leaves. Where the large, round marks of the cones are present, these are an infallible guide, never being present in Sigillaria. As the cones grew on the upper sides of the branches, the impression of the lower side often shows no cone-scars, or only two lateral rows, whereas on the upper side of the same branch they appear spirally arranged. I may describe as an example--

Lepidophloios Acadianus, Dawson. Leaf-bases broadly rhombic, or in old stems regularly rhombic, prominent, ascending, terminated by very broad rhombic scars having a central point and two lateral obscure points. Outer bark laminated or scaly. Surface of inner bark with single points or depressions. Leaves long, linear, with a strong keel on one side, five inches or more in length. Cone-scars sparsely scattered on thick branches, either in two rows or spirally, both modes being sometimes seen on the same branch. Scalariform axis scarcely an inch in diameter in a stem five inches thick. Fruit, an ovate strobile with numerous acute scales covering small globular spore-cases. This species is closely allied to Ulodendron majus and Lepidophloios laricinus, and presents numerous varieties of marking. Coal-formation, Nova Scotia.

Family Calamiteæ; Genus Calamites, Suckow.

The plants of this genus are unquestionably allied to the modern Equisetaceæ, but excel these so much in variety of form and structure, and are so capricious in their states of preservation, and so liable to be mistaken for parts of plants generically different, that they have given rise to much controversy. The following considerations will enable us to arrive at some certainty.

The genus Calamites was originally founded in the longitudinally ribbed and jointed stems so frequent in the coal-formation, and of which the common C. Suckovii is a typical form. The most perfect of these stems represent the outer surface immediately within the epidermis, in which case transverse lines or constrictions mark the nodes, and at the nodes there are rounded spots, sometimes indicating radial processes of the pith, first described by Williamson; in other cases, the attachment of branchlets, or in some specimens both. But some specimens show the outer surface of the epidermis, in which case the transverse nodal lines are usually invisible, though the scars of branchlets may appear. In still other examples the whole of the outer tissues have perished, and the so-called Calamite is a cast of the interior of the stem, showing merely longitudinal ribbing and transverse nodal constrictions. In studying these plants in situ in the erect Calamite brakes of the coal-formation of Nova Scotia, one soon becomes familiar with these appearances, but they are evidently unknown to the majority of palæobotanists, though described in detail more than twenty years ago.

When the outer surface is preserved it is sometimes seen to bear verticils of long needle-like leaves (C. Cistii), or of branchlets with secondary whorls of similar leaves (C. Suckovii and C. undulatus). No Calamite known to me bears broad one-nerved leaves like those of Asterophyllites and Annularia, though the larger stems of these plants have been described as Calamites, and the term Calamocladus has been used to include both groups. The base of the Calamite stem usually terminates in a blunt point, and may be attached to a rhizome, or several stems may bud out from each other in a group or stool. The roots are long and cylindrical, sometimes branching. The fruit consists of spikes of spore-cases, borne in whorls and subtended by linear floral leaves. To these strobiles the name Calamostachys has been given.

Williamson has shown that the stem of Calamites consists of a central pith or cavity of large size surrounded by a cylinder consisting of alternate wedges of woody and cellular matter, with vertical canals at the inner sides of the wedges, and slender medullary rays. The thick cellular wedges intervening between the woody wedges he calls primary medullary rays; the smaller medullary rays in the wedges, secondary medullary rays. There is thus a highly complex exogenous stem based on the same principle with the stem of a common Equisetum, but with much greater strength and complexity.

Williamson has also shown that there are different sub-types of these stems. More especially he refers to the three following:

(a) Calamites proper, which has the woody wedges of scalariform or barred tissue with thin medullary rays, and the thick primary medullary rays are cellular.

(b) Calamopitus has reticulated or multiporous tissue in the woody wedges with medullary rays, and the primary medullary wedges are composed of elongated cells.

(c) Calamodendron has the woody wedges of barred tissue as in a, with medullary rays, but has the intervening medullary wedges of an elongated tissue approaching to woody fibre, and also with medullary rays.

To these I would add a fourth type, which I have described, from the coal-formation of Nova Scotia.

"Quarterly Journal of the Geological Society," 1871.

(d) Eucalamodendron differs from Calamodendron in having true bordered pores or pseudo-scalariform slit-pored tissue, and corresponds to the highest type of calamitean stem.

I would also add that under a and b there are some species in which the woody cylinder is very thin in comparison to the size of the stem. In c and d the woody cylinder is thick and massive, and the stems are often large and nodose.

As an example of an ordinary Calamite in which the external surface and foliage are preserved, I may quote the following from my report on the "Flora of the Lower Carboniferous and Millstone Grit," 1873:

Calamites Undulatus, Brongniart.--This species is stated by Brongniart to be distinguished from the C. Suckovii, the characteristic Calamite of the middle coal-formation, by its undulated ribs marked with peculiar cellular reticulation. He suggests that it may be merely a variety of C. Suckovii, an opinion in which Schimper coincides; but since I have received large additional collections from Mr. Elder, containing not only the stems and branches, but also the leaves and rhizomes, I am constrained to regard it as a distinct though closely allied species.

The rhizomata are slender, being from one to two inches in diameter, and perfectly flattened. They are beautifully covered with a cellular reticulation on the thin bark, and show occasional round areoles marking the points of exit of the rootlets. I have long been familiar with irregular flattened stems thus reticulate, but have only recently been able to connect them with this species of Calamite.

The main stems present a very thin carbonaceous bark reticulated like the rhizomes. They have flat, broad ribs separated by deep and narrow furrows, and undulated in a remarkable manner even when the stems are flattened. This undulation is, however, perhaps an indication of vertical pressure while the plant was living, as it seems to have had an unusually thin and feeble cortical layer, and the undulations are apparently best developed in the lower part of the stem. At the nodes the ribs are often narrowed and gathered together, especially in the vicinity of the rounded radiating marks which appear to indicate the points of insertion of the branches. At the top of each rib we have the usual rounded areole, probably marking the insertion of a primary branchlet.

The branches have slender ribs and distant nodes, from which spring secondary branchlet s in whorls, these bearing in turn small whorls of acicular leaflets much curved upward, and which are apparently round in cross section and delicately striate. They are much shorter than the leaves of Calamites Suckovii, and are less dense and less curved than those of C. nodosus, which I believe to be the two most closely allied species.

Lesquereux notices this species as characteristic of the lower part of the Carboniferous in Arkansas.

It will be observed that I regard the striated and ribbed stems not as internal axes, but as representing the outer surface of the plants. This was certainly the case with the present species and with C. Suckovii and C. nodosus. Other species, and especially those which belonged to Calamodendron, no doubt had a smooth or irregularly wrinkled external bark; but this gives no good ground for the manner in which some writers on this subject confound Calamites with Calamodendra, and both with Asterophyllites and Sphenophyllum. With this no one who has studied these plants, rooted in their native soils, and with their appendages still attached, can for a moment sympathise. One of the earliest geological studies of the writer was a bed of these erect Calamites, which he showed to Sir C. Lyell in 1844, and described in the "Proceedings of the Geological Society" in 1851, illustrating the habit of growth as actually seen well exposed in a sandstone cliff. Abundant opportunities of verifying the conclusions formed at that time have since occurred, the results of which have been summed up in the figures in Acadian Geology, which, though they have been treated by some botanists as merely restorations, are in reality representations of facts actually observed.

On these subjects, without entering into details, and referring for these to the elaborate discussions of Schimper, Williamson, and McNab, and to my paper on the subject, "Journal of the Geological Society," vol. xxvii, p. 54, I may remark:

1. That the aërial stems of ordinary Calamites had a thin cortical layer, with lacunæ and fibrous bundles and multiporous vessels--the whole not differing much from the structure of modern Equiseta.

2. Certain arborescent forms, perhaps allied to the true Calamites, as well as possibly the old underground stems of ordinary species assumed a thick-walled character in which the tissues resembled the wedges of an exogen, and abundance of pseudo-scalariform fibres were developed, while the ribbing of the external surface became obsolete or was replaced by a mere irregular wrinkling.

Williamson, "Transactions of the Royal Society." McNab, in "Proceedings of the Edinburgh Botanical Society."

3. Sufficient discrimination has not been exercised in separating casts of the internal cavities of Calamites and Calamodendron from those representing other surfaces and the proper external surface.

4. There is no excuse for attributing to Calamites the foliage of Annularia, Asterophyllites, and Sphenophyllum, since these leaves have not been found attached to true Calamite stems, and since the structure of the stems of Asterophyllites as described by Williamson, and that of Sphenophyllum as described by the writer, are essentially different from those of Calamites.

"Journal of the Geological Society," 1866.

5. As the species above described indicates, good external characters can be found for establishing species of this genus, and these species are of value as marks of geological age.

Genus Archæocalamites, Sternberg.

This genus has been established to include certain Calamites of the Devonian and Lower Carboniferous, in which the furrows on the stem do not alternate at the nodes or joints, and the leaves in one species at least bifurcate. C. radiatus, Brongniart, is the typical species. In North America it occurs in the Erian, probably as low as the Middle Erian. In Europe it has so far been recognised in the Lower Carboniferous only. I have, however, seen stems from alleged Devonian beds in Devonshire which may have belonged to this species.

Family Asterophylliteæ; Genus Asterophyllites, Brongniart.

Stems ribbed and jointed like the Calamites, but with inflated nodes and a stout internal woody cylinder, which has been described by Williamson. From the joints proceeded whorls of leaves or of branchlets, bearing leaves which differed from those of Calamites in their having a distinct middle rib or vein. The fructification consisted of long slender cones or spikes, having whorls of scales bearing the spore-cases. Some authors speak of Asterophyllites as only branches and leaves of Calamites; but though at first sight the resemblance is great, a close inspection shows that the leaves of Asterophyllites have a true midrib, which is wanting in Calamites.

Genus Annularia.--It is perhaps questionable whether these plants should be separated from Asterophyllites, The distinction is that they produce branches in pairs, and that their whorls of leaves are one-sided and usually broader than those of Asterophyllites, and united into a ring at their insertion on the stem. One little species, A. sphenophylloides, is very widely distributed.

Pinnularia--a provisional genus---includes slender roots or stems branching in a pinnate manner, and somewhat irregularly. They are very abundant in the coal shales, and were probably not independent plants, but aquatic roots belonging to some of the plants last mentioned. The probability of this is farther increased by their resemblance in miniature to the roots of Calamites. They are always flattened, but seem originally to have been round, with a slender thread-like axis of scalariform vessels, enclosed in a soft, smooth, cellular bark.

Family Rhizocarpeæ; Genus Sphenophyllum.

Leaves in whorls, wedge-shaped, with forking veins. Fructification on spikes, with verticils of sporocarps. These plants are by some regarded as allied to the Calamiteæ and Asterophylliteæ, by others as a high grade of Rhizocarps of the type of Marsilia. The stem had a star-shaped central bundle of scalariform or reticulato-scalariform vessels.

Genus Sporangites. (Sporocarpon, Williamson.)

Under this name we may provisionally include those rounded spherical bodies found in the coal and its accompanying beds, and also in the Erian, which may be regarded as Macrospores or Sporocarps of Protosalvinia, or other Rhizocarpean plants akin to those described above in Chapter III, which see for description.

Genus Protosalvinia.--Under this we include sporocarps allied to those of Salvinia, as described in Chapter III.

Family Filices.

Under this head I shall merely refer to a few groups of special interest, and to the provisional arrangement adopted for the fronds of ferns when destitute of fructification.

The external appearances of trunks of tree-ferns have been already referred to.

With respect to tree ferns, the oldest known examples are those from the Middle Devonian of New York and Ohio, which I have described in the "Journal of the Geological Society," 1871 and 1881. As these are of some interest, I have reproduced their descriptions in a note appended to