Part 12

b. Single joint of ray of Asterolepis.]

COPROLITES OF ASTEROLEPIS.

(Nat. Size.)]

On exactly the same grounds I infer that certain large coprolites of common occurrence in the Thurso flagstones, which contain the broken scales of Dipterians, and exhibit a curiously twisted form, (fig. 44,) also belonged to the Asterolepis; and from these, that the creature was carnivorous in its habits,—an inference which the character of its teeth fully corroborates; and farther, that, like the sharks and rays, and some of the extinct Enaliosaurs, it possessed the spiral disposition of intestine. Paley, in his chapter on the compensatory contrivances palpable in the structure of various animals, refers to a peculiar substitutory provision which occurs in a certain amphibious animal described in the Memoirs of the French Academy. “The reader will remember,” he says, “what we have already observed concerning the intestinal canal,—that its length, so many times exceeding that of the body, promotes the extraction of the chyle from the aliment, by giving room for the lacteal vessels to act upon it through a greater space. This long intestine, whenever it occurs, is in other animals disposed in the abdomen from side to side, in returning folds. But in the animal now under our notice, the matter is managed otherwise. The same intention is mechanically effectuated, but by a mechanism of a different kind. The animal of which I speak is an amphibious quadruped, which our authors call the Alopecias or sea-fox. The intestine is straight from one end to the other but in this straight, and consequently short intestine, is a winding, cork-screw, spiral passage, through which the food, not without several circumvolutions, and, in fact, by a long route, is conducted to its exit. Here the shortness of the gut is compensated by the obliquity of the perforation.” This structure of intestine, which all the true Placoids possess, and at least the Sturiones among existing Ganoids, seems to have been an exceedingly common one during both the Palæozoic and Secondary periods. It has left its impress on all the better preserved coprolites of the Coal Measures, so abundant in the shales of Newhaven and Burdie House, and on those of the Lias and Chalk. It seems to be equally a characteristic of well nigh all the bulkier coprolites of the Lower Old Red Sandstone. In these, however, it manifests a peculiar trait, which I have failed to detect in any of the recent fishes; nor have I yet seen it indicated, in at least the same degree, by the Carboniferous or Secondary coprolitic remains. In the bowels which moulded the coprolites of Lyme-Regis, of the Chalk, and of the Newhaven and Granton beds, a single screw must have winded within the cylindrical tube, as a turnpike stair winds within its hollow shaft; and such also is the arrangement in the existing Sharks and Rays; whereas the bowels which moulded the coprolites of the Lower Old Red Sandstone must have been traversed by triple or quadruple screws laid closely together, as we find the stalk of an old-fashioned wine-glass traversed by its thickly-set spiral lines of thread-like china. And so, while on the surface of both the Secondary and Carboniferous coprolites there is space between the screw-like lines for numerous cross markings that correspond to the thickly set veiny branches which traverse the sides of the recent placoid bowel, the entire surface of the Lower Old Red coprolites is traversed by the spiral markings. Is there nothing strange in the fact, that after the lapse of mayhap millions of years,—nay, it is possible, millions of ages,—we should be thus able to detect at once general resemblance and special dissimilarity in even the most perishable parts of the most ancient of the Ganoids?

I must advert, in passing, to a peculiarity exemplified in the state of keeping of the bones of this ancient Ganoid, in at least the deposits of Orkney and Caithness. The original animal matter has been converted into a dark-colored bitumen, which in some places, where the remains lie thick, pervades the crevices of the rocks, and has not unfrequently been mistaken for coal. In its more solid state it can hardly be distinguished, when used in sealing a letter,—a purpose which it serves indifferently well,—from black wax of the ordinary quality; when more fluid, it adheres scarce less strongly to the hands than the coal-tar of our gas-works and dock-yards. Underneath a specimen of Asterolepis, first pointed out to me in its bed among the Thurso rocks by Mr. Dick, and which, at my request, he afterwards raised and sent me to Edinburgh, packed up in a box, there lay a quantity of thick tar, which stuck as fast to my fingers, on lifting out the pieces of rock, as if I had laid hold of the planking of a newly tarred yawl. What had been once the nerves, muscles, and blood of this ancient Ganoid still lay under its bones, and reminded me of the appearance presented by the remains of a poor suicide, whose solitary grave, dug in a sandy bank in the north of Scotland, had been laid open by the encroachments of a river. The skeleton, with pieces of the dress still wrapped round it, lay at length along the section; and, for a full yard beneath, the white dry sand was consolidated into a dark-colored pitchy mass, by the altered animal matter which had escaped from it, percolating downwards, in the process of decay.

In consequence of the curious chemical change which has thus taken place in the animal juices of the Asterolepis, its remains often occur in a state of beautiful preservation: the pervading bitumen, greatly more conservative in its effects than the oils and gums of an old Egyptian undertaker, has maintained, in their original integrity, every scale, plate, and bone. They may have been much broken ere they were first committed to the keeping of the rock, or in disentangling them from its rigid embrace; but they have, we find, caught no harm when under its care. Ere the skeleton of the Bruce, disinterred after the lapse of five centuries, was recommitted to the tomb, such measures were taken to secure its preservation, that, were it to be again disinterred, even after as many more centuries had passed, it might be found retaining unbroken its gigantic proportions. There was molten pitch poured over the bones, in a state of sufficient fluidity to permeate all the pores, and fill up the central hollows, and which, soon hardening around them, formed a bituminous matrix, in which they may lie unchanged for a thousand years. Now, exactly such was the process to which nature resorted with these gigantic skeletons of the Old Red Sandstone. Like the bones of the Bruce, they are bones steeped in pitch; and so thoroughly is every pore and hollow still occupied, that, when cast into the fire, they flame like torches. Though black as jet, they still retain, too, in a considerable degree, the peculiar qualities of the original substance. The late Mr. George Sanderson of Edinburgh, one of the most ingenious lapidaries in the kingdom, and a thoroughly intelligent man, made several preparations for me, for microscopic examination, from the teeth and bones; and though they were by far the oldest vertebrate remains he had ever seen, they exhibited, he informed me, in the working, more of the characteristics of recent teeth and bone than any other fossils he had ever operated upon. Recent bone when in the course of being reduced on the wheel to the degree of thinness necessary to secure transparency, is apt, under the heat induced by the friction, to acquire a springy elasticity, and to start up from the glass slip to which it has been cemented; whereas bone in the fossil state usually lies as passive, in such circumstances, as the stone which envelopes it. Mr. Sanderson was, however, surprised to find that the bone of the Asterolepis still retained its elasticity, and was scarce less liable, when heated, to start from the glass,—a peculiarity through which he at first lost several preparations. I have seen a human bone that had for ages been partially embedded in a mass of adipocere, partially enveloped in the common mould of a churchyard, exhibit two very different styles of keeping. In the adipocere it was as fresh and green as if it had been divested of the integuments only a few weeks previous; whereas the portion which projected into the mould had become brittle and porous, and presented the ordinary appearance of an old churchyard bone. And what the adipocere had done for the human bone in this case, seems to have been done for the bones of the Asterolepis by the animal bitumen.

HYOID PLATE OF THURSO ASTEROLEPIS.

(One fifth the nat. size, linear.)]

The size of the Asterolepis must, in the larger specimens, have been very great. In all those ganoidal fishes of the Old Red Sandstone that had the head covered with osseous plates, we find that the cranial buckler bore a certain definite proportion,—various in the several genera and species,—to the length of the body. The drawing-master still teaches his pupils to regulate the proportions of the human figure by the seven head-lengths which it contains; and perhaps shows them how an otherwise meritorious draftsman, much employed half an age ago in drawing for the wood-engraver, used to render his figures squat and ungraceful by making them a head too short. Now, those ancient Ganoids which possessed a cranial buckler may, we find, be also measured by head-lengths. Thus, in the Coccosteus decipiens, the length of the cranial buckler from nape to snout equalled one fifth the entire length of the creature from snout to tail. The entire length of the Glyptolepis was equal to about five one half times that of its cranial buckler. The Pterichthys was formed in nearly the same proportions. The Diplopterus was fully seven times the length of its buckler: and the Osteolepis from six and a half to seven. In all the cranial bucklers of the Asterolepis yet found, the snout is wanting. The very fine specimen figured in page 99 (fig. 28) terminates abruptly at the little plate between the eyes, the specimen figured in page 98 (fig. 27) terminates at the upper line of the eye. The terminal portion which formed the snout is wanting in both, and we thus lack the measure, or module, as the architect might say, by which the proportions of the rest of the creature were regulated. We can, however, very nearly approximate to it. A hyoid plate in my collection (fig. 45) is, I find, so exactly proportioned in size to the cranial buckler, (fig. 28,) that it might have belonged to the same individual; and by fitting it in its proper place, and then making the necessary allowance for the breadth of the nether jaw, which swept two thirds around it, and was surmounted by the snout, we ascertain that the buckler, when entire, must have been, as nearly as may be, a foot in length. If the Asterolepis was formed in the proportions of the Coccosteus, the buckler (fig. 28) must have belonged to an individual five feet in length; if in the proportions of the Pterichthys or Glyptolepis, to an individual five and a half feet in length; and if in those of the Diplopterus or Osteolepis, to an individual of from six and a half to seven feet in length. Now I find that the hyoid plate can be inscribed—such is its form—in a semicircle, of which the nail-shaped ridge in the middle (if we strike off a minute portion of the sharp point, usually wanting in detached specimens) forms very nearly the radius, and of which the diameter equals the breadth of the cranial buckler, along a line drawn across at a distance from the nape, equal to two thirds of the distance between the nape and the eyes. Thus, the largest diameter of a hyoid plate which belonged to a cranial buckler a foot in length is, I find, equal to seven one quarter inches, while the length of its nape somewhat exceeds three five eighth inches. The nail of the Stromness specimen measures five and a half inches. It must have run along a hyoid plate eleven inches in transverse breadth, and have been associated with a cranial buckler eighteen one eighth inches in length; and the Asterolepis to which it belonged must have measured from snout to tail, if formed, as it probably was, in the proportions of its brother Cœlacanth the Glyptolepis, eight feet three inches; and if in those of the Diplopterus, from nine feet nine to ten feet six inches. This oldest of Scottish fish—this earliest-born of the Ganoids yet known—was at least as bulky as a large porpoise.

It was small, however, compared with specimens of the Asterolepis found elsewhere. The hyoid plate figured in page 110, (fig. 36,)—a Thurso specimen which I owe to the kindness of Mr. Dick,—measures nearly fourteen inches, and the cranial buckler of the same individual, fifteen one fourth inches, in breadth. The latter, when entire, must have measured twenty-three one half inches in length; and the fish to which it belonged, if formed in the proportions of the Glyptolepis, ten feet six inches; and if in those of the Diplopterus, from twelve feet five to thirteen feet eight inches in length. Did the shield still exist in its original state as a buckler of tough, enamel-crusted bone, it might be converted into a Highland target, nearly broad enough to cover the ample chest of a Rob Roy or Allan M’Aulay, and strong enough to dash aside the keenest broadsword. Another hyoid plate found by Mr. Dick measures sixteen one half inches in breadth; and a cast in the British Museum, from one of the Russian specimens of Professor Asmus, (fig. 46,) twenty-four inches. The individual to which this last plate belonged must, if built in the shorter proportions, have measured eighteen, and if in the longer, twenty-three feet in length. The two hyoid plates of the specimen of Holoptychius in the British Museum measure but four and a half inches along that transverse line in which the Russian Asterolepis measures two feet, and the largest Thurso specimen sixteen inches and a half. The maxillary bone of a cod-fish two and a half feet from snout to tail measures three inches in length. One of the Russian maxillary bones in the possession of Professor Asmus measures in length twenty-eight inches. And that space circumscribed by the sweep of the lower jaw which it took, in the Russian specimen, a hyoid plate twenty-four inches in breadth to fill, could be filled in the two-and-a-half-feet cod by a plate whose breadth equalled but an inch and a half. Thus, in the not unimportant circumstance of size, the most ancient Ganoids yet known, instead of taking their places, agreeably to the demands of the development hypothesis, among the sprats, sticklebacks, and minnows of their class, took their place among its huge basking sharks, gigantic sturgeons, and bulky sword-fishes. They were giants, not dwarfs.

HYOID PLATE OF RUSSIAN ASTEROLEPIS.

(One twelfth the natural size, linear.)]

But what of their organization? Were they fishes low or high in the scale? On this head we can, of course, determine merely by the analogies which their structure exhibits to that of fishes of the existing period; and these point in three several directions;—in two of the number, directly on genera of the high Ganoid order; and in the third, on the still higher Placoids and Enaliosaurs. No trace of vertebræ has yet been found; and so we infer—lodging, however, a precautionary protest, as the evidence is purely negative, and therefore it some degree inconclusive—that the vertebral column of the Asterolepis was, like that of the sturgeon, cartilaginous. Respecting its external covering, we positively know, as has been already shown, that, like the Lepidosteus of America and the Polypterus of the Nile, it was composed of strong plates and scales of solid bone; and, regarding its dentition, that, as in these last genera, and even more decidedly than in these, it was of the mixed ichthyic-reptilian character,—an outer row of thickly-set fish-teeth being backed by an inner row of thinly-set reptile-teeth. And its form of coprolite indicates the spiral disposition of intestine common to the Rays and Sharks of the existing period, and of the Ichthyosauri of the Secondary ages. Instead of being, as the development hypothesis would require, a fish low in its organization, it seems to have ranged on the level of the highest ichthyic-reptilian families ever called into existence. Had an intelligent being, ignorant of what was going on upon earth during the week of creation, visited Eden on the morning of the sixth day, he would have found in it many of the inferior animals, but no trace of man. Had he returned again in the evening, he would have seen, installed in the office of keepers of the garden, and ruling with no tyrant sway as the humble monarchs of its brute inhabitants, two mature human creatures, perfect in their organization, and arrived at the full stature of their race. The entire evidence regarding them, in the absence of all such information as that imparted to Adam by Milton’s angel, would amount simply to this, that in the morning man was not, and that in the evening he was. There, of course, could not exist, in the circumstances, a single appearance to sanction the belief that the two human creatures whom he saw walking together among the trees at sunset had been “developed from infusorial points,” not created mature. The evidence would, on the contrary, lie all the other way. And in no degree does the geologic testimony respecting the earliest Ganoids differ from what, in the supposed case, would be the testimony of Eden regarding the earliest men. Up to a certain point in the geologic scale we find that the Ganoids are not; and when they at length make their appearance upon the stage, they enter large in their stature and high in their organization.

FISHES OF THE SILURIAN ROCKS—UPPER AND LOWER. THEIR RECENT HISTORY, ORDER, AND SIZE.

But the system of the Old Red Sandstone represents the second, not the first, great period of the world’s history. There was a preceding period at least equally extended, perhaps greatly more so, represented by the Upper and Lower Silurian formations. And what is the testimony of this morning period of organic existence, in which, so far as can yet be shown, vitality, in the planet which man inhabits, and of whose history or productions he knows anything, was first associated with matter? May not the development hypothesis find a standing in the system representative of this earliest age of creation, which it fails to find in the system of the Old Red Sandstone?

It has been confidently asserted, not merely that it may, but that it does. Ever since the publication, in 1839, of Sir Roderick Murchison’s great work on the Silurian System, it had been known that the remains of fishes occur in a bed of the “Ludlow Rock,”—one of the most modern deposits of the Upper Silurian division; and subsequent discoveries both in England and America, had shown that even the base of this division has its ichthyic organisms. But for year after year, the lower half of the system,—a division more than three thousand feet in thickness,—had failed, though there were hands and eyes busy among its deposits, to yield any vertebrate remains. During the earlier half of the first great period of organic existence, though the polyparia, radiata, articulata, and mollusca, existed, as their remains testified, by myriads, fish had, it was held, not yet entered upon the scene; and the assertors of the development theory founded largely on the presumed fact of their absence. “It is still customary,” says the author of the “Vestiges of Creation,” in his volume of “Explanations,” “to speak of the earliest fauna as one of an elevated kind. When rigidly examined, it is not found to be so. IN THE FIRST PLACE, IT CONTAINS NO FISH. There were seas supporting crustacean and molluscan life, but utterly devoid of a class of tenants who seem able to live in every example of that element which supports meaner creatures. This single fact, that only invertebrated animals now lived, is surely in itself a strong proof that, in the course of nature, time was necessary for the creation of the superior creatures. And if so, it undoubtedly is a powerful evidence of such a theory of development as that which I have presented. If not, let me hear an equally plausible reason for the great and amazing fact, that seas were for numberless ages destitute of fish. I fix my opponents down to the consideration of this fact, so that no diversion respecting high molluscs shall avail them.” And how is this bold challenge to be met?

Most directly, and after a fashion that at once discomfits the challenger.

It might be rationally enough argued in the case, that the author of the “Vestiges” was building greatly more on a piece of purely negative evidence,—the presumed absence of fish from the Lower Silurian formations,—than purely negative evidence is, from its nature as such, suited to bear; that only a very few years had passed since it was known that vertebrate remains occurred in the Upper Silurian, and only a few more since they had been detected in the Old Red Sandstone; nay, that within the present century their frequent occurrence in even the Coal Measures was scarce suspected; and that, as his argument, had it been founded twelve years ago on the supposed absence of fishes from the Upper Silurian, or twenty years ago on the supposed absence of fishes from the Old Red Sandstone, would have been quite as plausible in reference to its negative data then as in reference to its negative data now, so it might now be quite as erroneous as it assuredly would have been then. Or it might be urged, that the fact of the absence of fish from the Lower Silurians, even were it really a fact, would be in no degree less reconcilable with the theory of creation by direct act, than with the hypothesis of gradual development. The fact that Adam did not exist during the first, second, third, fourth, and fifth days of the introductory week of Scripture narrative, furnishes no argument whatever against the fact of his creation on the sixth day. And the remark would of course equally apply to the non-existence of fishes during the Lower Silurian period, had they been really non-existent at the time, and to their sudden appearance in that of the Upper. But the objection admits of a greatly more conclusive answer. “I fix my opponents down,” says the author of the “Vestiges,” “to the consideration of this fact,” i. e. that of the absence of fishes from the earliest fossiliferous formations. And I, in turn, fix you down, I reply, to the consideration of the antagonist fact, not negative, but positive, and now, in the course of geological discovery, fully established, that fishes were not absent from the earliest fossiliferous formations. From none of the great geological formations were fishes absent,—not even from the formations of the Cambrian division. “The Lower Silurian,” says Sir Roderick Murchison, in a communication with which, in 1847, he honored the writer of these chapters, “is no longer to be viewed as an invertebrate period; for the Onchus (species not yet decided) has been found in the Llandeilo Flags and in the Lower Silurian rocks of Bala. In one respect I am gratified by the discovery; for the form is so very like that of the Onchus Murchisoni of the Upper Ludlow rock, that it is clear the Silurian system is one great natural-history series, as is proved, indeed, by all its other organic remains.” It may be mentioned further, in addition to this interesting statement, that the Bala spine was detected in its calcareous matrix by the geologists of the Government Survey, and described to Sir Roderick as that of an Onchus, by a very competent authority in such matters,—Professor Edward Forbes, and that the annunciation of the existence of spines of fishes in the Llandeilo Flags we owe to one of the most cautious and practised geologists of the present age,—Professor Sedgwick of Cambridge.

So much for the fact of the existence of vertebrata in the Lower Silurian formations, and the argument founded on their presumed absence. Let me now refer—their presence being determined—to the tests of size and organization. Were these Silurian fishes of a bulk so inconsiderable as in any degree to sanction the belief that they had been developed shortly before from microscopic points? Or were they of a structure so low as to render it probable that their development was at the time incomplete? Were they, in other words, the embryos and fœtuses of their class? or did they, on the contrary, rank with the higher and larger fishes of the present time?

It is of importance that not only the direct bearing, but also the actual amount, of the evidence in this case, should be fairly stated. So far as it extends, the testimony is clear; but it does not extend far. All the vertebrate remains yet detected in the Silurian System, if we except the debris of the Upper Ludlow bone-bed, might be sent through the Post-Office in a box scarcely twice the size of a copy of the “Vestiges.” The naturalist of an exploring party, who, in crossing some unknown lake, had looked down over the side of his canoe, and seen a few fish gliding through the obscure depths of the water, would be but indifferently qualified, from what he had witnessed, to write a history of all its fish. Nor, were the some six or eight individuals of which he had caught a glimpse to be of small size, would it be legitimate for him to infer that only small-sized fish lived in the lake; though, were there to be some two or three large ones among them, he might safely affirm the contrary. Now, the evidence regarding the fishes of the Silurian formation very much resembles what that of the naturalist would be, in the supposed case, regarding the fishes of the unexplored lake; with, however, this difference, that as the deposits of the ancient system in which they occur have been examined for years in various parts of the world, and all its characteristic organisms, save the ichthyic ones, found in great abundance and fine keeping, we may conclude that the fish of the period were comparatively few. The palæontologist, so far as the question of number is involved, is in the circumstances, not of the naturalist who has only once crossed the unknown lake, but of the angler who, day after day, casts his line into some inland sea abounding in shell-fish and crustacea, and, after the lapse of months, can scarce detect a nibble, and, after the lapse of years, can reckon up all the fish which he has caught as considerably under a score. The existence of this great division of the animal kingdom, like that of the earlier reptiles during the Carboniferous period, did not form a prominent characteristic of those ages of the earth’s history in which they began to be.

The earliest discovered vertebral remains of the system—those of the Upper Ludlow rock—were found in digging the foundations of a house at Ludford, on the confines of Shropshire, and submitted, in 1838, by Sir Roderick Murchison to Agassiz, through the late Dr. Malcolmson of Madras. I used at the time to correspond on geological subjects with Dr. Malcolmson,—an accomplished geologist and a good man, too early lost to science and his friends,—and still remember the interest which attached on this occasion to his communication bearing the Paris post-mark, from which I learned for the first time that there existed ichthyic fragments greatly older than even the ichthyolites of the Lower Old Red Sandstone, and which made me acquainted with Agassiz’s earliest formed decision regarding them. Though existing in an exceedingly fragmentary condition,—for the materials of the thin dark-colored layer in which they had lain seemed as if they had been triturated in a mortar,—the ichthyologist succeeded in erecting them into six genera; though it may be very possible,—as some of these were formed for the reception of detached spines, and others for the reception of detached teeth,—that, as in the case of Dipterus and Asterolepis, the fragments of but a single genus may have been multiplied into two genera or more. And minute scale-like markings, which mingled with the general mass, and were at first regarded as the impressions of real scales, have been since recognized as of the same character with the scale-like markings of the Seraphim of Forfarshire, a huge crustacean. Even admitting, however, that a set of teeth and spines, with perhaps the shagreen points represented in page 54, fig. 2, b, in addition, may have all belonged to but a single species of fish, there seem to be materials enough, among the remains found, for the erection of two species more. And we have evidence that at least two of the three kinds were fishes of the Placoid order, (Onchus Murchisoni and Onchus tenuistriatus,) and—as the supposed scales must be given up—no good evidence that the other kind was not. The ichthyic remains of the Silurian System next discovered were first introduced to the notice of geologists by Professor Phillips, at the meeting of the British Association in 1842. They occurred, he stated, in a quarry near Hales End, at the base of the Upper Ludlow rock, immediately over the Aymestry Limestone, and were so exceedingly diminutive, that they appeared to the naked eye as mere discolored spots; but resolved under the microscope into scattered groupes of minute spines, like those of the Cheiracanthus, with what seemed to be still more minute scales, or, perhaps,—what in such circumstances could scarce be distinguished from scales,—shagreen points of the scale-like type. The next ichthyic organism detected in the Silurian rocks occurred in the Wenlock Limestone, a considerably lower and older deposit, and was first described in the “Edinburgh Review” for 1845 by a vigorous writer and masterly geologist, (generally understood to be Professor Sedgwick of Cambridge,) as “a characteristic portion of a fish undoubtedly belonging to the Cestraciont family of the Placoid order.” In the “American Journal of Science” for 1846, Professor Silliman figured, from a work of the States’ Surveyors, the defensive spine of a Placoid found in the Onondago Limestone of New York,—a rock which occurs near the base of the Upper Silurian System, as developed in the western world; and in the same passage he made reference to a mutilated spine detected in a still lower American deposit,—the Oriskany Sandstone. In the Geological Journal for 1847, it was announced by Professor Sedgwick, that he had found “defences of fishes” in the Upper Llandeilo Flags, and by Sir Roderick Murchison, that the “defence of an Onchus” had been detected by the geologists of the Government survey, in the Limestone near Bala. Sir Roderick referred in the same number to the remains of a fish found by Professor Phillips in the Wenlock Shale. And such, up to the present time, is the actual amount of the evidence with which we have to deal, and the dates of its piecemeal production. Let us next consider the order of its occurrence in the geologic scale.

{ +-----+ { Upper | | Fish, 1838, { Ludlow. | 1 | (Murchison.) { | | Fish, 1842, { +-----+ (Phillips.) { Aymestry | | { Limestone. | 2 | { +-----+ UPPER SILURIAN ROCKS. { Lower | | { Ludlow. | 3 | { +-----+ { Wenlock | | Fish, 1845, { Limestone. | 4 | (Sedgwick.) { | | Fish, 1846, { +-----+ (Silliman.) { Wenlock | 5 | Fish, 1847. { Shale. | | (Phillips.) +-----+ ----- { +-----+ { Caradoc | | { Sandstone, | 6 | { &c. | | LOWER SILURIAN ROCKS. { +-----+ { Llandeilo | | Fish, 1847, { Flags, &c. | 7 | (Sedgwick.) { +-----+ ----- { +-----+ { Plynlimmon | | { Group. | a | { +-----+ { Bala | | Fish, 1847, CAMBRIAN ROCKS. { Limestone. | b | (Geologists of { | | Government { +-----+ Survey.) { Snowdon | | { Group. | c | Fucoids. { | | { +-----+

The better marked sub-divisions of the Silurian System, as described in the great work specially devoted to it, may be regarded as seven in number. An eight has since been added, by the transference of the Tilestones from the lower part of the Old Red Sandstone group, to the upper part of the Silurian group underneath; but in order the better to show how ichthyic discovery has in its slow course penetrated into the depths, I shall retain the divisions recognized as those of the system when that course began. The highest or most modern Silurian deposit, then, (No. 1 of the accompanying diagram,) is the Upper Ludlow Rock; and it is in the superior strata of this division that the bone-bed discovered in 1838 occurs; while the exceedingly minute vertebrate remains described by Professor Phillips in 1842 occur in its base. The division next in the descending order is the Aymestry Limestone, (No. 2;) the next (No. 3.) the Lower Ludlow rock; then (No. 4.) the Wenlock or Dudley Limestone occurs; and then, last and oldest deposit of the Upper Silurian formation, the Wenlock shale, (No. 5.) It is in the fourth, or Wenlock Limestone division, that the defensive spine described in the “Edinburgh Review” for 1845 as the oldest vertebrate organism known at the time, was found; while the vertebrate organism found by Professor Phillips belongs to the fifth, or base deposit of the Upper Silurian. Further, the American spines of Onondago and Oriskany, described in 1846, occurred in rocks deemed contemporary with those of the Wenlock division. We next cross the line which separates the base of the Upper from the top of the Lower Silurian deposits, and find a great arenaceous formation, (No. 6,) known as the Caradoc Sandstones; while the Llandeilo Flags, (No. 7,) the formation upon which the sandstones rest, compose, according to the sections of Sir Roderick, published in 1839, the lowest deposit of the Lower Silurian rocks. And it is in the upper part of this lowest member of the system that the ichthyic defences, announced in 1847 by Professor Sedgwick, occur. Vertebrate remains have now been detected in the same relative position in the seventh and most ancient member of the system, that they were found to occupy in its first and most modern member ten years ago. But this is not all. Beneath the Lower Silurian division there occur vast fossiliferous deposits, to which the name “Cambrian System” was given, merely provisionally, by Sir Roderick, but which Professor Sedgwick still retains as representative of a distinct geologic period; and it is in these, greatly below the Lower Silurian base line, as drawn in 1839, that the Bala Limestones occur. The Plynlimmon rocks (a)—a series of conglomerate, grauwacke, and slate beds, several thousand yards in thickness—intervene between the Llandeilo Flags and the Limestones of Bala, (b.) And, of consequence, the defensive spine of the Onchus, announced in 1847 as detected in these limestones by the geologists of the Government Survey, must have formed part of a fish that perished many ages ere the oldest of the Lower Silurian formations began to be deposited.

Let us now, after this survey of both the amount of our materials, and the order and time of their occurrence, pass on to the question of size, as already stated. Did the ichthyic remains of the Silurian System, hitherto examined and described, belong to large or to small fishes? The question cannot be altogether so conclusively answered as in the case of those Ganoids of the Lower Old Red Sandstone whose dermal skeletons indicate their original dimensions and form. In fishes of the Placoid order, such as the Sharks and Rays, the dermal skeleton is greatly less continuous and persistent than in such Ganoids as the Dipterians and Cœlacanths; and when their remains occur in the fossil state, we can reason, in most instances, regarding the bulk of the individuals of which they formed part, merely from that of detached teeth or spines, whose proportion to the entire size of the animals that bore them cannot be strictly determined. We can, indeed, do little more than infer, that though a large Placoid may have been armed with but small spines or teeth, a small Placoid could not have borne very large ones. And to this Placoid order all the Silurian fish, from the Aymestry Limestone to the Cambrian deposits of Bala inclusive, unequivocally belong. Nor, as has been already said, is there sufficient evidence to show that any of the ichthyic remains of the Upper Ludlow rocks do not belong to it. It is peculiarly the order of the system. The Ludlow bone-bed contains not only defensive spines, but also teeth, fragments of jaws, and shagreen points; whereas, in all the inferior deposits which yield any trace of the vertebrata, the remains are those of defensive spines exclusively. Let us, then, take the defensive spine as the part on which to found our comparison.

One of the best marked Placoids of the Upper Ludlow bone-bed is that Onchus Murchisoni to which the distinguished geologist whose name it bears refers, in his communication, as so nearly resembling the oldest Placoid yet known,—that of the Bala Limestone. And the living fishes with which the Onchus Murchisoni must be compared, says Agassiz, though “the affinity,” he adds, “may be rather distant,” are those of the genera “Cestracion, Centrina, and Spinax.” I have placed before me a specimen of recent Spinax, of a species well known to all my readers on the sea-coast, the Spinax Acanthias, or common dog-fish, so little a favorite with our fishermen. It measures exactly two feet three inches in length; and of the defensive spines of its two dorsals,—these spear-like thorns on the creature’s back immediately in advance of the fins, which so frequently wound the fisher’s hand,—the anterior and smaller measures, from base to point, an inch and a half, and the posterior and larger, two inches. I have also placed before me a specimen of Cestracion Phillippi, (the Port Jackson Shark,) a fish now recognized as the truest existing analogue of the Silurian Placoids. It measures twenty-two three fourth inches in length, and is furnished, like Spinax, with two dorsal spines, of which the anterior and larger measures from base to point one one half inch, and the posterior and smaller, one one fifth inch. But the defensive spine of the Onchus Murchisoni, as exhibited in one of the Ludlow specimens, measures, though mutilated at both ends, three inches and five eighth parts in length. Even though existing but as a fragment, it is as such nearly twice the length of the largest spine of the dog-fish, unmutilated and entire, and considerably more than twice the length of the largest spine of the Port Jackson Shark. The spines detected by Professor Phillips, in an inferior stratum of the same upper deposit, were, as has been shown, of microscopic minuteness; and when they seemed to rest on the extreme horizon of ichthyic existence as the most ancient remains of their kind, the author of the “Vestiges” availed himself of the fact. He regarded the little creatures to which they had belonged is the fœtal embryos of their class, or—to employ the language of the Edinburgh Reviewer—as “the tokens of Nature’s first and half-abortive efforts to make fish out of the lower animals.” From the latter editions of his work, the paragraph to which the Reviewer refers has, I find, been expunged; for the horizon has greatly extended, and what seemed to be its line of extreme distance has travelled into the middle of the prospect. But that the passage should have at all existed is a not uninstructive circumstance, and shows how unsafe it is, in more than external nature, to regard the line at which, for the time, the landscape closes, and heaven and earth seem to meet, as in reality the world’s end. The Wenlock spine, though certainly not microscopic, is, I am informed by Sir Philip Egerton, of but small size; whereas the contemporary spine of the Onondago Limestone, though comparatively more a fragment than the spine of the Upper Ludlow Onchus,—for it measures only three inches in length,—is at least five times as bulky as the largest spine of Spinax Acanthias. Representing one of the massier fishes disporting amid the some four or five small ones, of which in my illustration, the naturalist catches a glimpse in fording the unknown lake, it at least serves to show that all the Silurian ichthyolites must not be described as small, seeing that not only might many of its undetected fish have been large, but that some of those which have been detected were actually so. Another American spine, of nearly the same formation,—for it occurs in a limestone, varying from twenty to seventy feet in thickness, which immediately overlies that of the Onondago deposit, though still more fragmentary than the first, for its length is only two three eighth inches,—maintains throughout a nearly equal thickness,—a circumstance in itself indicative of considerable size; and in positive bulk it almost rivals the Onondago one. Of the Lower Silurian and Bala fishes no descriptions or figures have yet appeared. And such, up to the present time, is the testimony derived from this department of Geology, so far as I have been able to determine it, regarding the size of the ancient Silurian vertebrata. “No organism,” says Professor Oken, “is, nor ever has one been, created, which is not microscopic.” The Professor’s pupils and abettors, the assertors of the development hypothesis, appeal to the geological evidence as altogether on their side in the case; and straightway a few witnesses enter court. But, lo! among the expected dwarfs, there appear individuals of more than the average bulk and stature.

a. Posterior Spine of Spinax Acanthias.

b. Fragment of Onondago Spine.

(Natural Size.)]