CHAPTER VIII. Trilobita

TRILOBITA

Among the many interesting groups of fossils found in the Palaeozoic deposits there is none which has attracted more attention than the Trilobites. As early as 1698, Edward Lhwyd, Curator of the Ashmolean Museum in Oxford, recorded in the Philosophical Transactions the discovery of Trilobites in the neighbourhood of Llandeilo in South Wales; and of one of his specimens he remarked that “it must be the Sceleton of a flat Fish.” In the following year the same writer gave in his Lithophylacii Britannici Ichnographia descriptions and figures of two Trilobites which are evidently examples of the species now known as Ogygia buchi and Trinucleus fimbriatus.

Although Trilobites differ so much from living Arthropods that it was difficult to determine even whether they belonged to the Crustacea or the Arachnida, yet one of the earliest writers, Dr. Cromwell Mortimer, Secretary of the Royal Society (1753), recognised their resemblance to Apus (see pp. 19–36). This view of their affinities was adopted by Linnaeus, and has been supported by many later writers. Another early author, Emanuel Mendez da Costa, thought that the Trilobites were related to the Isopods, an opinion which has been held by some few zoologists of more recent times.

The Trilobites form the only known Order of the Crustacea which has no living representatives. They are found in the oldest known fossiliferous deposits—the Lower Cambrian or Olenellus beds, where they are represented by 19 genera belonging to the families Agnostidae, Paradoxidae, Olenidae, and Conocephalidae. From the variety of forms found and the state of development which they have reached, it is evident that even at that remote period the group must have been of considerable antiquity; but of its pre-Cambrian ancestors nothing is yet known; consequently there is no direct evidence of the origin of the group.

Trilobites form an important part of all the faunas of the Cambrian system; they attain their greatest development in the Ordovician period, after which they become less numerous; their decline is very marked in the Devonian, in which nearly all the genera are but survivals from the Silurian period; in the Carboniferous, evidence of approaching extinction is seen in the small number of genera represented, all of which belong to one family—the Proëtidae, in the relatively few species in each genus and in the small size of the individuals of those species. In Europe no representatives of the group appear to have survived the Carboniferous period, but in America one form has been recorded from deposits of Permian age.

Trilobites seem to have been exclusively marine, since they are found only in association with the remains of marine animals. Their range in depth was evidently considerable, for they occur in many different kinds of sediment, and were apparently able to live regardless of the nature of the sea-floor—whether muddy, sandy, calcareous, or rocky. In some cases they occur in deposits containing reef-building corals and other shallow water animals; in others they are associated with organisms which lived at greater depths. The group appears to have had a world-wide distribution, for the remains of Trilobites are found in the Palaeozoic rocks of all countries. Their range in size is considerable; for whilst a large proportion of the species are about two or three inches in length, some, like Agnostus, are only a quarter of an inch long, others are from ten to twenty inches long, the largest forms including species of Paradoxides, Asaphus, Megalaspis, Lichas, and Homalonotus.

The feature in a Trilobite which first attracts attention is the marked division of the dorso-ventrally flattened body into a median or axial part, and a lateral or pleural part on each side. It was this character that led Walch, in 1771, to give the name by which the group is now known. The axial part of the body contained the alimentary canal, as is shown by the position of the mouth and anus, as well as by casts in mud of the canal which are found in some specimens. The trilobation of the body is quite distinct in the majority of Trilobites, but in a few genera belonging to the Asaphidae and Calymenidae (Fig. 136) it becomes more or less completely obsolete.

FIG. 136.—Homalonotus delphinocephalus, Green, × 1. Silurian. (After Zittel.) ]

In most cases the only part of the Trilobite which is preserved is the exoskeleton which covered the dorsal surface of the body. That skeleton consists largely of calcareous material, and shows in sections a finely perforated structure. Generally it is arched above, but in some cases is only slightly convex; in outline it is more or less oval. Three regions can always be distinguished in the body of a Trilobite—the head, the thorax, and the abdomen or pygidium.

The carapace which covers the =head= is known as the cephalic shield (Fig. 137, A, 1), and is commonly more or less semicircular in outline, but varies considerably in different genera. Only in a few cases, as in some species of Agnostus (Fig. 146), is its length greater than its breadth. The axial part of the cephalic shield, called the “glabella” (Fig. 137, A, a), is usually more convex than the lateral parts (“cheeks” or “genae”), and is separated from them by longitudinal or axial furrows (b). The shape of the glabella varies greatly; it may be oblong, circular, semi-cylindrical, pyriform, spherical, etc. Its relative size likewise varies; thus in Phacops cephalotes it expands in front and forms the larger part of the head, whilst in Arethusina (Fig. 151, B) it is narrow and short, being only about one-half of the length of the head.

FIG. 137.—Calymene tuberculata, Brünn. × 1. Silurian, Dudley. =A=, Dorsal surface: 1, head; 2, thorax; 3, pygidium or abdomen. a, Glabella; b, axial furrow; c, glabella-furrow; d, neck-furrow; e, fixed cheek; f, free cheek; g, facial suture; h, eye; i, genal angle; k, axis of thorax; l, pleura. =B=, Ventral surface of head (after Barrande): a, hypostome; b, doublure; c, c′, facial sutures; d, rostral suture; e, rostral plate. =C=, One segment of the thorax: a, ring of axis; b, groove; c, articular portion; d, axial furrow; d-f, pleura; d-e, internal part of pleura; e-f, external part of pleura; e, fulcrum; g, groove. =D=, Coiled specimen: a, glabella; b, eye; c, facial suture; d, pygidium; e, rostral suture; f, continuation of facial suture. ]

The segmentation of the head is indicated by transverse furrows on the glabella (Fig. 137, A, c, d). In some cases these furrows extend quite across the glabella (Fig. 147), but commonly they are found on the sides only and divide the glabella into lateral lobes. Only the posterior or “neck-furrow” (Fig. 137, A, d) is continued on to the cheeks, and the segment which it limits anteriorly on the glabella is known as the occipital or neck-ring. In front of the neck-furrow there may be three other furrows, so that altogether five cephalic segments are indicated by the furrows of the glabella. Commonly all the furrows are distinct in the primitive types; but in the more modified forms some, especially the anterior, become either reduced in size or obsolete. The actual number of furrows present consequently varies in different genera, and may even differ in different species of the same genus. In a few genera all the furrows are either indistinct or absent, as for example in Ellipsocephalus (Fig. 150, B). In some cases four furrows are present in addition to the neck-furrow; this is due to the division of the anterior lobe of the glabella by fulcra which are developed for the attachment of muscles.

When the glabella reaches the front border of the head the two cheeks are separated (Fig. 150, I); but in other cases they unite in front of the glabella (Fig. 150, C). The outer posterior angle of the cheeks or genae (“genal angle,” Fig. 137, A, i) may be rounded, pointed, or produced into backwardly directed spines (Fig. 140). The marginal part of the cephalic shield is often flattened or concave; this border may be quite a narrow rim as in Calymene (Fig. 137, A), but in some genera (e.g. Trinucleus, Fig. 140, B; Harpes, Fig. 150, A; Asaphus) it attains a great development. Each cheek is usually divided by a suture— the “facial suture” (Fig. 137, A, g)—into an inner and an outer part; the former is the “fixed cheek” (e), and the latter the “free cheek” (f). The course of the facial suture varies in different genera: on the posterior part of the head it begins either at the posterior margin (Fig. 150, C) or at the posterior part of the lateral margin (Fig. 151, C, D); at first it is directed inwards, and then bends forward, forming an angle. In front it may (a) end at the front margin (Fig. 147), or (b) be united beneath the front margin by a rostral suture (Fig. 137, B, d, D, e), or (c) unite with the other suture on the dorsal surface in front of the glabella (Fig. 151, C). In the last case the free cheeks also unite in front of the glabella.

The facial suture is one of the distinguishing features of the Trilobites, and may have been of some use in ecdysis. In only a few forms is it absent, as for example in Agnostus (Fig. 146) and Microdiscus. In the former, however, Beecher states that a suture is really present, but, unlike that of most other Trilobites, it is situated at the margin of the cephalic shield, and consequently the free cheek, if present, must be on the ventral surface. Lindström and Holm, after a re-examination of well-preserved specimens, deny the existence of a suture in Agnostus. By most authors Olenellus is said to be without a suture, but Beecher maintains that although the fixed and free cheeks have coalesced, yet a raised line passing from the eye-lobe to the posterior margin marks the position of the suture; this view is not accepted by Lindström.

The existence of a facial suture in Trinucleus has likewise been disputed. But Emmerich, Salter, and M‘Coy have maintained that a suture is present in a normal position on the dorsal surface, extending from the posterior margin just within the genal angle to the eye (when present), and from thence bending forward and ending on the front margin near the glabella. It must be admitted that no indications of the suture are seen in the majority of specimens, perhaps owing to the fact that most examples of Trinucleus are in the form of internal casts; perhaps also to the more or less complete coalescence of the fixed and free cheeks, since in no specimen has the free cheek been found separated from the rest of the head, as occurs not uncommonly in many other Trilobites. The probability of the existence of a suture receives some support from the fact that one is found in the allied genera Orometopus and Ampyx (Fig. 140). Barrande and Oehlert deny its existence in Trinucleus. There is, however, in that genus a suture running close to the margin of the cephalic border, and joining the genal angle so as to cut off the genal spine. Lovén and Oehlert claim that this suture represents the facial suture, but in an abnormal position; this view, however, is not accepted by Beyrich. In this connection it should be noted that in Acidaspis, whilst the majority of the species possess a facial suture, there are two in which it has disappeared owing to the fusion of the fixed and free cheeks. Such being the case, it seems not improbable that the curved line passing backwards from the eye in Harpes may mark the position of the suture; but it is stated that the only suture present in that form runs at the margin of the cephalic border, and is similar to that of Trinucleus. This matter will be referred to again when discussing the nature of the eyes in Trinucleus and Harpes.

The relative sizes of the fixed and free cheeks obviously depend on the position of the facial suture; when this starts on the lateral margin of the cephalic shield and passes forward to the outer part of the front margin, the free cheek will be a narrow strip; when, on the other hand, the suture starts from the posterior margin and runs close to the glabella, the free cheek will be relatively large and the fixed cheek narrow. The fixed cheek is small in Phacops, Cheirurus, and Illaenus; relatively large in Remopleurides, Phillipsia, and Stygina. It was suggested by M‘Coy that the free cheek represents the pleura of an anterior segment which has not become fused with the other cephalic pleurae. The fixed cheek appears to be formed of the coalesced pleurae of the other cephalic segments, but of those pleurae the only indication seen in adult specimens is in the neck-ring; in young specimens of Olenellus, however, the presence of other pleurae is indicated by furrows on the cheeks in front of the neck-furrow.

FIG. 138.—Phacops latifrons, Bronn, × 1. Devonian. Showing large compound eye. (After Zittel.) ]

A pair of compound =eyes= are present in the majority of Trilobites. Each eye is situated on the free cheek, at that part of its inner margin where the facial suture bends to form an angle (Figs. 137, A, h, 138). The position of the eye is consequently determined by the position of the facial suture; it may be near the glabella or near the lateral margin of the head, and either as far forward as the first segment of the glabella or nearly as far back as the neck-furrow. In many Trilobites the eye is more or less conical, with its summit truncated or rounded, but in some genera it is ovoid, or crescentic. In Aeglina (Fig. 150, H) the eye is flattened and scarcely raised above the general level of the cheek. The eye of a Trilobite is oriented so that its longer axis is parallel or nearly parallel to the axis of the body (Fig. 150, G); but in one case (Encrinurus intercostatus) it is placed at right angles to this axis. The size of the eye varies considerably; it is largest in Aeglina, in which it covers nearly the whole of the free cheek; it is small in Acidaspis and Encrinurus.

Though the eye is always entirely on the free cheek, the adjoining part of the fixed cheek is raised to form a buttress on which the eye rests; this buttress, which is known as the “palpebral lobe,” is seen clearly when the fixed cheek is removed. The eyes of Trilobites are always sessile; for although in some species, such as Asaphus cornigerus, A. kowalewskii, and Encrinurus punctatus, they are on the summits of prominent stalks, yet those stalks are immovable.

Three types of compound eye have been recognised in Trilobites— holochroal, prismatic, and schizochroal.

FIG. 139.—Eyes of Trilobites. (After Lindström.) =A=, =B=, Sphaerophthalmus alatus, Ang. Upper Cambrian. Vertical and horizontal sections, × 100. =C=, Asaphus fallax, Dalm. Horizontal section, × 60. =D=, Nileus armadillo, Dalm. Vertical section, × 60, a, prismatic lenses; b, cuticle; c, part of free cheek. =E=, Dalmanites vulgaris, Salt. Part of eye, × 30. =F=, Dalmanites imbricatulus, Ang. Vertical section of eye, with a part of the free cheek on the left, × 60. =G=, =H=, Harpes vittatus, Barr. =G=, The two lenses of one eye, × 8; =H=, vertical section of the same, × 60. ]

1. In the holochroal eye (Fig. 139, A, B) the lenses are globular or biconvex and close together, so that the cornea is continuous over the entire eye. Examples of this are seen in Bronteus and Sphaerophthalmus.

2. In the prismatic type (Fig. 139, C, D) the lenses are prismatic and plano-convex, and the entire surface of the eye is covered by a smooth cuticle. The lenses are close together and usually hexagonal, but occasionally rhombic or square. Near the margin of the eye the lenses may become irregular, giving rise to a border in which the prismatic structure is more or less indistinct. The prismatic type of eye is found in the genera Asaphus, Nileus, Illaenus, etc.

3. The schizochroal eye occurs only in the family Phacopidae (Fig. 139, E, F). The lenses are biconvex and are separated by portions of the cephalic shield, so that each lens appears to rest in a separate socket, and the cornea is not continuous for the entire eye. The lenses are circular in outline, but owing to the upward and inward growth of the interstitial test they may appear, on the surface, to be hexagonal. The diameter of a lens may be as much as 0·5 mm. The crystalline cones have not been preserved. In specimens of Phacops rana, in which the inner face of the lens is more convex than the outer, J. M. Clarke has obtained evidence of a posterior spheroidal cavity in addition to the anterior corneal cavity. The complete separation of the lenses in this type of eye has led to the suggestion that the schizochroal eye is an aggregate rather than a compound eye. But the difference between this and the holochroal eye is probably less than appears at first sight if the statement made by Clarke is confirmed, namely, that in young specimens of Calymene senaria the lenses are relatively large and similar to those of Phacops, whereas in the adult the eye is holochroal.

These three types of eye, according to Lindström, have appeared successively in chronological order: the prismatic occurring first in the Olenus beds (Upper Cambrian), the holochroal first in the Ceratopyge Limestone (Uppermost Cambrian), and the schizochroal first in the Ordovician. The number of lenses in the eye varies greatly. For example, in Trimerocephalus volborthi there are 14 only, whilst in Remopleurides radians there are as many as 15,000. Even in different species of the same genus there may be considerable differences. Thus Bronteus brongniarti possesses 1000, B. palifer 4000, lenses in each eye. The number increases from the young up to the adult, but decreases in old age. The lenses are usually arranged in alternating rows. In Trilobites with a conical eye the outer segment of the cone bears the visual surface. It has been stated that the eyes of Trilobites resemble those of Isopods, but close comparison is difficult to make, since in Trilobites no part of the eye beneath the lenses is preserved. In some genera a threadlike ridge, called the “eye-line,” passes from the glabella, generally from the front segment, to the eye, where it often ends in the palpebral lobe; this eye-line is found in nearly all genera which are confined to the Cambrian period, and persists in a few of later date, as for example in Triarthrus, Euloma, and some species of Calymene from the Ordovician; in Arethusina and Acidaspis from the Silurian; and in Harpes from the Devonian (Fig. 150, A).

FIG. 140.—Trinucleidae. =A=, Orometopus elatifrons, Ang. × 5. Restoration based on specimens from the Upper Cambrian (Tremadoc) of Shineton, Shropshire. =B=, Trinucleus bucklandi, Barr. Ordovician, Bohemia. A complete but not fully-grown individual showing eyes. Natural size. (After Barrande.) =C=, Ampyx rouaulti, Barr. × 3. Ordovician, Bohemia. (After Barrande.) ]

In Harpes and in some species of Trinucleus eyes are present, but have been stated to be of a different type. They are described as simple eyes, and have been compared with ocelli; they are never found in Trilobites which possess the compound eyes described above. In Harpes (Fig. 150, A) the eye is near the middle of the cheek, in the position where compound eyes occur in other genera; it appears to consist of two or three granules or tubercles which are really lenses, and is connected with the front of the glabella by an eye-line. No facial suture can be seen, consequently the whole of the cheek is stated to be the fixed cheek. In Trinucleus (Fig. 140, B) a single tubercle is found on the middle of the cheek in the young of some species, and is sometimes connected with the glabella by an eye-line; the latter disappears before the adult state is reached, and in some species the tubercle also disappears, but in others (such as T. seticornis, T. bucklandi) it persists in the adult individuals.

From the lateral position of these eyes they can hardly be compared with the median simple eye of other Crustacea. In Harpes it is more probable that, as suggested by J. M. Clarke, they are schizochroal eyes imperfectly developed. Their structure (Fig. 139, G, H) is somewhat similar to that of schizochroal eyes, and moreover, in one species, H. macrocephalus, there are, in addition to the three main tubercles, other smaller tubercles in regular rows. Further, the eye-line occupies the same position as in other Trilobites which have undoubted compound eyes. The absence of a facial suture cannot be taken as evidence against these eyes being of the ordinary type, since in some species of Acidaspis (e.g. A. verneuili, A. vesiculosa) which possess compound eyes there is, in consequence of the coalescence of the fixed and free cheeks, no suture.

In some species of Trinucleus (Fig. 140, B) the simple eye is found in the same position as the eye in Harpes, and if, as some writers have maintained, there is evidence of the existence of a suture in that genus, then there is no reason for regarding the eye as other than a degenerate form of compound eye. The probability of its being such is supported by the existence of a compound eye in a similar position in the allied form Orometopus (Fig. 140, A) which possesses a facial suture.

In some species of Trinucleus (Fig. 140, B) and Ampyx there is a small median tubercle on the front part of the glabella, which from its position may be a simple unpaired eye, but its structure appears to be unknown.

Some Trilobites possess no eyes. Well-known examples of such are Agnostus, Microdiscus, Ampyx, Conocoryphe, and some species of Illaenus and Trinucleus; such blind Trilobites are almost confined to the Cambrian and Ordovician periods. All the forms of later periods, with the exception of a species of Ampyx, and possibly one or two other species, possess eyes. In addition to those undoubtedly blind forms Lindström considers that most of the Olenidae and Paradoxidae were without eyes. Many of the members of these families possess a lobe closely resembling a palpebral lobe, and a corresponding excavation in the free cheek; such forms have been generally regarded as possessing eyes; and the absence of any indication of lenses in those cases, on which Lindström lays stress, has been explained by the comparatively imperfect preservation of these early Trilobites. The development of the supposed eye-lobe in some of the Paradoxidae and Olenidae differs from that of the eyes in other families of Trilobites. In the latter the eye appears first at the margin of the head and always in connexion with the facial suture. But in Olenellus, in which there is said to be no facial suture, development shows that the crescentic eye-like lobe (Fig. 145, E) is really of the nature of a pleura coming off from the base of the first segment of the glabella. In Paradoxides, which resembles Olenellus in many respects, a facial suture is present and forms the outer boundary of the eye-like lobe, but it is developed subsequently to the appearance of the latter, which seems to be similar to that of Olenellus. In some genera of the Olenidae the eye-line, which comes off from the first segment of the glabella, ends in some cases in a swelling or knob which has hitherto been regarded as a palpebral lobe, but according to Lindström’s view no trace of an eye has been found in connexion with that lobe, nor is there any space between the lobe and the free cheek in which the eye could have occurred. If this view is correct it follows that the majority of the Cambrian Trilobites were blind. The earliest genus with eyes would then be Eurycare found in the Olenus beds of the Upper Cambrian. Sphaerophthalmus and Ctenopyge, found in the higher beds of the Cambrian, also possessed eyes, but Olenus and Parabolina were probably blind.

On the ventral surface of the head there is a flat rim around the margin (Fig. 137, B, b); this rim or “doublure” is the reflexed border of the cephalic shield. In many Trilobites its median part in front is cut off by sutures so as to form a separate plate (e); such is the case when the two facial sutures (c, c′) cut the anterior margin of the cephalic shield and are continued across the doublure, where they are joined by a transverse or rostral suture (d) just below the margin. When, however, as in Phacops and Remopleurides, the two facial sutures unite on the dorsal surface, in front of the glabella, the median part of the doublure is not separated from the lateral parts, or from the dorsal part of the cephalic shield.

FIG. 141.—=A=, Hypostome of Bronteus polyactin, Ang. showing maculae, × 4. =B=, Left macula of Bronteus irradians, Lindst. × 12. (After Lindström.) ]

The “labrum” or “hypostome” is attached to the doublure in front (Fig. 137, B, a); it is commonly an oval or shield-shaped plate, but is occasionally nearly square. Its surface is sometimes divided into two or three areas by shallow transverse grooves (Fig. 141, A), Just behind the middle of the hypostome, or when transverse grooves are present either in or near the anterior groove, there are often found a pair of small patches or “maculae” which are more or less oval or elliptical in outline (Fig. 141). The maculae may be (1) surrounded by a raised border, or (2) in the form of pits, or (3) raised like tubercles. In some cases the entire surface of a macula is smooth and glossy; in others either the whole or a part is covered with granules, and in the latter case the granules may be limited to the internal third (Fig. 141, B) or to the central portion. Sections of a macula show that the granules are really globular lenses similar to those of the compound eyes on the dorsal surface of the head. Some of the maculae which are without lenses show no structure, but in others there is a spongy or irregularly polyhedric structure with prisms, resembling the marginal zone of the prismatic eyes of some genera. There seems no doubt that the maculae with lenses are visual organs, and those without are degenerate eyes. They occur in some genera which, according to Lindström, are without eyes on the dorsal surface. Maculae do not appear to be present in other Crustacea, but they have been compared with a median organ, found just in front of the hypostome in Branchipus. Maculae, have so far been found in 136 species of Trilobites belonging to 39 genera ranging from Lower Cambrian to Carboniferous.

A “metastoma” or lower lip plate (Fig. 142, Ep) is found just behind the hypostome in Triarthrus, but has not been noticed in any other genus. Between the hypostome and the metastoma lies the mouth.

The segments of the =thorax= are free, and their number varies from two in Agnostus (Fig. 146) to twenty-six in Harpes (Fig. 150, A). In the Trilobites confined to the Cambrian period the number (except in the Agnostidae) is usually larger than in the genera found in the Ordovician and later periods. Owing to the free thoracic segments many Trilobites were able to curl up somewhat after the manner of a Wood-louse (Figs. 137, D, 138). The axial part of each thoracic segment is more or less considerably arched. Usually it consists of three parts: (i.) the largest part (Fig. 137, C, a), called the ring, is band-like in form, and is always visible whether the Trilobite is extended or coiled up; (ii.) in front of the ring is a depressed, groove-like part (Fig. 137, C, b) separating it from (iii.) the articular portion (c) which is convex in front and extends beneath the ring of the preceding segment; this part is only visible when the Trilobite is coiled up or when the segments are separated. In some few genera the axial part consists of a simple arched band without either a groove or a specially modified articular portion. The pleurae (Fig. 137, A, l, C, d-f) are fixed firmly to the axis, and have the form of narrow bands with the ends rounded, obtuse, pointed, or spinose. In a few cases the pleurae have a plain surface; but usually they possess either a ridge or a groove (Fig. 137, C, g); the former is generally parallel to the margins of the pleura, the latter is generally oblique, being inclined backwards from the axis. Sometimes in front of the ridge there is a small groove. On the ventral surface each pleura shows, at its outer extremity, a reflexed margin or doublure. At some distance from the axis the pleurae are bent downwards and backwards. The point where this bend occurs is called the “fulcrum” (e); it divides the pleura into an internal and an external part: the internal part (d-e) is flat or slightly convex, and just touches the front and back margins of the adjacent pleurae; the external part (e-f) may be (i.) narrower than the internal part, so that it is separated from the previous and succeeding pleurae; such occurs principally in pleurae with ridges, as in Cheirurus and Bronteus; or (ii.) it may be in the form of a long cylindrical process, as in many species of Acidaspis; or (iii.) the external part may be of the same width, either throughout or in part, as the internal part, and may overlap the next pleura behind; this type is found principally in pleurae with a groove such as in Phacops, Calymene, Sao, Asaphus, Ellipsocephalus.

In some Trilobites there is beyond the fulcrum a smooth, flat, triangular part at the front margin of the pleura; this part is known as the “facet,” and forms a surface articulating with the preceding segment which overlaps it.

In the remarkable form Deiphon (Fig. 151, E) the pleurae are separate throughout their entire length.

In some Trilobites broad and narrow forms of the same species occur—the difference being seen especially in the axis. The former are regarded as females, the latter as males.

The segments of the =abdomen= or =pygidium= (Fig. 137, A, 3) are similar to those of the thorax, except that they are fused together. In a few forms, such as Illaenus (Fig. 150, F) and Bumastus, the fusion is so complete that no trace of segmentation can be seen on the dorsal surface. Usually, however, the segments are easily distinguishable; the number seen on the axis is commonly greater than on the lateral parts of the pygidium; this difference is particularly well shown in Encrinurus. In Trilobites which have grooved pleurae the conspicuous grooves seen on the lateral parts of the pygidium are the grooves of the pleurae, the sutures between the pleurae being less distinct. The shape of the pygidium may be semicircular, a segment of a circle, trapezoidal, triangular, semi-parabolic, etc.; its size varies considerably; in the Cambrian forms it is usually small, but in the Trilobites of later periods it becomes relatively larger. The number of segments in the pygidium varies from two to twenty-eight. The axis of the pygidium tapers more rapidly than that of the thorax; sometimes it reaches quite to the posterior end of the body, but is commonly shorter than the pygidium; in Bronteus it is extremely short, and the grooves on the lateral parts of the pygidium radiate from it in a fan-like manner. Occasionally, as in Bumastus, the axis cannot be distinguished from the lateral parts. In a few early Trilobites (Olenellus, Holmia, Fig. 148, Paradoxides, Fig. 147) the lateral parts of the pygidium are very small. In some genera, such as Asaphus, the marginal part of the pygidium forms a flattened or concave border. The margin may be entire or produced into spines, and sometimes (Fig. 151, C) a caudal spine comes off from the end of the axis. On the ventral surface of the pygidium there is a marginal rim similar to the doublure of the cephalic shield. The anus is on the ventral surface of the last segment of the pygidium.

Although Trilobites are often found in abundance and in an excellent state of preservation, it is only in very rare cases that anything is seen of the ventral surface except the hypostome and the reflexed borders of the cephalic shield, of the thoracic segments, and of the pygidium. The usual absence of =appendages= is probably due to their tenuity. Billings, in 1870, first obtained clear evidence of the presence of pairs of appendages, in Asaphus platycephalus. Soon afterwards Walcott showed their existence in American specimens of Asaphus megistos, Calymene senaria, and Cheirurus pleurexacanthus. In the two latter species the appendages were found by cutting sections of curled-up specimens obtained from the Trenton Limestone; 2200 examples were sliced, of which 270 showed evidence of the existence of appendages. They were seen to be present on the head, thorax, and pygidium; a ventral uncalcified cuticle with transverse arches was also found. By means of sections of curled-up specimens it was difficult to determine satisfactorily the form and position of the appendages. Subsequently extended specimens of Triarthrus (Fig. 142) and Trinucleus, showing the ventral surface and appendages clearly, were discovered in the Utica Slate (Ordovician) near Rome, New York. A full account of the appendages in those specimens has been given by Beecher.

FIG. 142.—Triarthrus becki, Green, × 2½. Utica Slate (Ordovician), near Rome, New York. =A=, Ventral surface with appendages; Ep, metastome; Hy, hypostome. =B=, second thoracic appendage; en, endopodite; ex, exopodite, × 12. (After Beecher.) ]

In Triarthrus each segment, except the anal, bears a pair of appendages, all of which, except the first, are biramous. There are five pairs of cephalic appendages; the first pair are attached at each side of the hypostome, and have the structure of antennae, each consisting of a single flagellum formed of short conical joints. The other cephalic appendages increase in size successively. At present the second and third pairs are not satisfactorily known, but appear to have been similar to the fourth and fifth pairs. The second pair is attached at the level of the posterior end of the hypostome. The fourth and fifth pairs have large, triangular coxopodites which served as gnathobases, their inner edges being denticulate; the endopodites consist of stout joints; the exopodites are slender, and bear setae which are often arranged in a fan-like manner.

The first pair of appendages appear to be antennules, whilst the second pair probably represent the antennae, the third pair the mandibles, and the fourth and fifth pairs the maxillae of other Crustacea. The appendages of the thorax and pygidium do not differ essentially from the two posterior cephalic appendages. Those on the anterior part of the thorax are the longest; the others gradually decrease in size in passing posteriorly. Each thoracic leg (Fig. 142, B) consists of a short coxopodite with an inward cylindrical prolongation forming a gnathobase which is best developed on the anterior legs; the endopodite and exopodite are long and nearly equal; the former consists of six joints tapering gradually to the end; the latter, of a long proximal joint with a denticulate edge and a distal part of ten or more joints, and it bears a row of setae along the whole of the posterior edge.

The anterior appendages of the pygidium differ but little from the posterior thoracic legs; but the phyllopodous character, which appears in the latter, becomes more distinct in the appendages of the pygidium, especially those near its posterior end, and is due to the broad, flat, laminar joints of the endopodite.

The more striking features of the appendages of Triarthrus are the small amount of differentiation which they show in different parts of the body, especially the want of specialisation in the cephalic region; the distinctly biramous character of all except the first pair; and the presence of one pair of functional antennae only, and the occurrence of thoracic gnathobases.

In Trinucleus the appendages are not so well known, but they are considerably shorter than in Triarthrus.

In the Palaeozoic rocks of Bohemia, where Trilobites are very perfectly preserved, Barrande discovered the larval forms of several species, and in some cases was able to trace out the =development= very completely, but in others the earliest stages were not found. In the strata in which Trilobites occur Barrande found minute spheroidal bodies, usually of a black colour, and only a little smaller than the youngest larval stages; those bodies are probably the eggs of Trilobites. Since the publication of Barrande’s work the development of some species found in North America has been studied by Ford, Matthew, Walcott, and Beecher. But even now the development is known in only a very small proportion of the total number of genera of Trilobites. The early larval form (Fig. 143, A) is similar in general character in the various species in which it has been found. It is circular or ovoid in outline, with a length of from 0·4 to 1 mm., and consists of a large cephalic and a small pygidial portion; the axis is distinct and usually shows more or less clear indications of five cephalic segments; the eyes, when present, are found at or near the front margin, and the free cheeks, if visible at all on the dorsal surface, are narrow. For this early larval form Beecher has proposed the name “protaspis”; he regards it as the representative of the Nauplius of other Crustacea, but that view is not accepted by Professor J. S. Kingsley.

The general changes which occur in the course of development are: modifications in the shape and relative size of the glabella, and of the number and depth of the glabella-furrows; the growth of the free cheeks and the consequent inward movement of the facial sutures and eyes; the introduction of and gradual increase in number of the thoracic segments, and the relative decrease in size of the head.

FIG. 143.—Development of Sao hirsuta, Barr. Cambrian. =A=, Protaspis; =B-F=, later stages; =G=, adult. The small outlines below each figure show the actual size of each specimen. (After Barrande.) ]

Sao hirsuta is a species found in the Cambrian, the development of which was fully described by Barrande. Its earliest protaspis stage (Fig. 143, A) is circular in outline; the glabella expands in front and reaches the anterior margin; the pygidial region is not distinctly separated from the cephalic region; segmentation is indicated in the former, and the neck-ring is present in the latter; the eye-line is seen on each side of the glabella near the anterior margin. In a later stage (Fig. 143, C) the segmentation of the glabella becomes more distinct, indicating the existence of five cephalic segments, and the facial suture appears near the margin limiting a very narrow free cheek. Subsequently (Fig. 143, D-F) the thoracic segments develop, and increase in number until the adult stage (G) is reached; also the eyes appear at the margin of the cephalic shield, and gradually move inwards, and the glabella becomes narrower and rounded in front, and ceases to reach the anterior margin. In this species the eye-line is present in the adult.

FIG. 144.—Triarthrus becki, Green. Ordovician. =A=, =B=, Two successive stages of the protaspis, × 45. (After Beecher.) ]

In the protaspis of Triarthrus (Fig. 144), found in the Ordovician, the glabella does not reach the front margin nor expand in front as it does in Sao; an eye-line is present, but disappears before the adult stage is reached.

FIG. 145.—Larval stages of Trilobites. =A-D=, Dalmanites socialis, Barr. Ordovician, Bohemia. The small figures below show the natural size of each specimen. (After Barrande.) =E=, Mesonacis asaphoides, Emmons, × 10. Lower Cambrian, North America. (After Walcott.) =F=, Acidaspis tuberculata, Conrad, × 20. Lower Helderberg Group (Lower Devonian or Upper Silurian), Albany County. (After Beecher.) ]

Dalmanites (Fig. 151, C) is a more advanced type than Sao and Triarthrus, and is found in later deposits. In the earliest stage (Fig. 145, A) the head and pygidium are quite distinct, and there is no eye-line present at this or any stage in development, but large ovoid eyes are found on the front margin, and have their long axes placed transversely to the axis of the body; the glabella is strongly segmented and is rounded in front. In later stages (C, D) the pygidium increases in size relatively, and the thoracic segments are successively introduced; the facial sutures and free cheeks appear on the dorsal surface, and as the free cheeks grow the eyes move inwards and backwards, and gradually swing round until their long axes become parallel with the axis of the body.

The larval form of Acidaspis (Fig. 145, F) is of interest since even in the earliest stage it shows the spiny character which forms such a striking feature of the adult (Fig. 151, F).

Before the discovery of the ventral surface of Trilobites it was thought by some zoologists that their =affinities= were with the Xiphosura rather than with the Crustacea. But the presence of antennae, and of five pairs of cephalic appendages; the biramous thoracic and pygidial appendages, the hypostome, and the character of the larval form, as well as the absence of a genital operculum, separate the Trilobites from the Xiphosura and connect them with the Crustacea.

The position of the Trilobites in the Crustacea is, however, difficult to determine. Already in the Cambrian period, at least five main groups of the Crustacea were clearly differentiated, namely, the Phyllopoda, Ostracoda, Cirripedia, Trilobita, and Leptostraca (Phyllocarida), and probably also the Copepoda, but of the last no remains have been preserved as fossils. Palaeontology, therefore, furnishes no connecting links between any two of these orders.

The Crustacea to which the Trilobites show some resemblance are the families Apodidae and Branchipodidae of the Order Phyllopoda (see pp. 19–36). The Trilobita agree with those families in having a large but variable number of trunk-segments, in the possession of a large labrum (hypostome), and in the occurrence of gnathobases on the thoracic appendages; also the foliation of some of the trunk-appendages is somewhat similar. The points of difference, however, are considerable; thus the cephalic appendages are much more specialised in the Apodidae and Branchipodidae than in the Trilobita; in the latter all, with the exception of the antennae, are distinctly biramous, and whilst the basal joints were masticatory the distal parts appear to have been locomotor organs. The appendages of the trunk also differ considerably; in the Trilobita all are clearly biramous, those of the thorax having a schizopodal form. In the possession of a single pair of antennae the Trilobita differ from other Crustacea; but in some forms of Apus the second pair of antennae may be rudimentary or even absent.

There are still other features which characterise the Trilobita: thus the eyes are borne on free cheeks, and differ in structure from those of Phyllopods. The broad pygidium formed of fused segments and without terminal fulcra is quite unlike the slender-jointed abdomen of Apus and Branchipus; and whilst in the Trilobites all the segments bear appendages, in the Phyllopods some, at any rate, of the posterior segments are devoid of appendages. The distinct division of the body into an axial and pleural region is not seen in Phyllopods, and is probably a character of some importance, since it occurs in the great majority of Trilobites, including all the early forms.

The existence of some relationship between the Trilobita and the Leptostraca (Phyllocarida) has been maintained by Professor G. H. Carpenter. He points out that some of the earliest Trilobites, such as Holmia kjerulfi (Fig. 148), possess nearly the same number of segments as Nebalia (Fig. 76, p. 111), and that in the latter genus the cephalic appendages, especially the mandibles and maxillae, are less specialised than in Apus, and consequently differ less from those of Trilobites than do the appendages of the Apodidae. Further, in another genus of the Leptostraca, Paranebalia, the biramous thoracic legs, in which both endopodite and exopodite are elongate, approach those of Trilobites more nearly than do the thoracic legs of Apus.

The view that some connexion may exist between the Isopoda and the Trilobita seems to have been based on the similar dorso-ventral flattening of the body, its division into three regions—head, thorax, and abdomen—and the presence of sessile eyes. Beyond this it is difficult to find any resemblance; whilst the differences, such as the variable number of thoracic segments and their biramous appendages in Trilobites, are important.

At present, then, we can only conclude that the Trilobita are more primitive than any other Crustacea, and that their resemblance to some of the Phyllopoda is sufficient to make it probable that they had some ancestral connexion; the possibility of such a relationship receives some support from the presence in the Lower Cambrian rocks of Protocaris, a genus of the Phyllopoda which resembles Apus. The primitive characters of Trilobites are the variable and often large number of segments in the thorax and pygidium; the presence of a pair of appendages on every segment except the anal; the biramous form of all except the first pair of appendages; and the lack of specialisation shown by the appendages, especially those of the head.

The =classification= of Trilobites is due largely to the work of Barrande and Salter, and the families defined by those authors have been, in the main, generally adopted. But the phylogenetic relationship of the families has still, to a large extent, to be established. Salter arranged the families in four groups, but did not claim that that classification was entirely natural. His groups with the families included in each are:—

1. Agnostini. Without eyes or facial suture. Agnostidae.

2. Ampycini. Facial sutures obscure, or submarginal, or absent. Eyes often absent. Trinucleidae.

3. Asaphini. Facial sutures ending on the posterior margin. Acidaspidae, Lichadidae, Harpedidae, Calymenidae, Paradoxidae, Conocephalidae, Olenidae, Asaphidae, Bronteidae, and Proëtidae.

4. Phacopini. Facial sutures ending on the lateral margins. Eyes well developed. Phacopidae, Cheiruridae, and Encrinuridae.

A modification of Salter’s classification has been brought forward by Beecher who divides the Trilobita into three main groups:—

1. Hypoparia. Facial sutures at or near the margin, or ventral. Compound eyes absent. This is equivalent to Salter’s Agnostini and Ampycini with the addition of the Harpedidae.

2. Opisthoparia. Facial sutures extending from the posterior margin to the front margin, but occasionally uniting in front of the glabella. Eyes holochroal or prismatic, but sometimes absent. This comprises the same families as Salter’s Asaphini with the exclusion of the Harpedidae and Calymenidae.

3. Proparia. Facial sutures extending from the lateral margins, and either cutting the anterior margin or uniting in front of the glabella. Eyes holochroal or schizochroal; occasionally absent. This is equivalent to Salter’s Phacopini with the addition of the Calymenidae.

In each of the groups proposed Beecher regards as the more primitive forms those which possess characters similar to those of the early larval stages, such as narrow free cheeks, the absence of compound eyes, and a glabella which is broad in front and reaches the anterior margin of the head.

The modifications introduced by Beecher can scarcely be regarded as making Salter’s classification more natural. For instance, the Agnostidae differ so much from all other families that, at present, there is no evidence to show that they have any close phylogenetic relationship with the Trinucleidae and Harpedidae. Further, the Calymenidae, which Salter recognised as related to the Olenidae, have been shown by the careful work of Professor Pompeckj to have descended from the latter family, and to have no genetic connexion with the Phacopidae with which they are grouped by Beecher. Then in the Trinucleidae the earliest genus, Orometopus (Fig. 140, A), possesses compound eyes and facial sutures which begin at the posterior margin and unite in front of the glabella; so that, according to Beecher’s classification, that genus would belong to the Opisthoparia, whereas the later genera (Trinucleus, etc.) of the same family would be placed in the Hypoparia. At present, therefore, the only classification of Trilobites which can be adopted is a division into families, of which a short account is given below.

FIG. 146.—Agnostus integer, Beyr., × 8. Cambrian. (After Barrande.) ]

=Fam. 1. Agnostidae= (Fig. 146).—Small Trilobites, in which the head and pygidium are of nearly the same size and shape. The thorax is shorter than the head or pygidium, and consists of from two to four segments with grooved pleurae. The length and width of the head are commonly nearly equal, but sometimes the length is greater. Eyes are absent. Facial sutures appear to be absent, but are stated by Beecher to be at the margin of the cephalic shield. From the absence of eyes, the probable absence of facial sutures, the few or indistinct furrows on the glabella, and the smaller number of thoracic segments, the Agnostidae appear to be degenerate forms. Microdiscus is apparently less modified than Agnostus, on account of the larger number of thoracic segments, the more distinct segmentation of the pygidium, and, in some species, the larger number of furrows on the glabella. Cambrian and Ordovician. Genera: Agnostus, Microdiscus.

=Fam. 2. Shumardiidae.=—The body is very small and oval. The cephalic shield is nearly semicircular and very convex, with a broad glabella which expands in front, and in which the furrows, except the neck-furrow, are indistinct. The facial suture is marginal and eyes are absent. There are six thoracic segments with ridged pleurae; the axis is broader than the pleurae. The pygidium is large, and is formed of about four segments similar to those of the thorax. Upper Cambrian and Ordovician. Genus: Shumardia.

=Fam. 3. Trinucleidae= (Fig. 140).—The head is large and has a flat border (except in Ampyx), and long genal spines. In the earliest genus (Orometopus) the facial sutures start from the posterior margin (near the genal angle) and pass obliquely inwards to the compound eye, from whence they continue forward and unite in front of the glabella. In Ampyx the suture starts from just within the genal angle and passes to the front border, cutting off a narrow free cheek; eyes are absent. In most specimens of Trinucleus no sutures are seen, but some examples show indications of what may be a facial suture (see p. 226), and a suture is sometimes found at the margin of the cephalic border; eyes may occur (see p. 230). The thorax consists of from five to eight segments, with grooved pleurae. The pygidium is triangular. Principally Ordovician. Genera: Orometopus (Upper Cambrian), Ampyx, Trinucleus, Dionide.

=Fam. 4. Harpedidae= (Figs. 139, G, H; 150, A).—The head is large and has a broad, flat border which is finely punctate, and extends backwards on each side in the form of a horn-like projection nearly as far as the posterior end of the thorax. The glabella is convex and does not reach the front margin. The cheeks are less convex than the glabella, and bear eyes which usually consist of two or three lenses. An eye-line connects the eye with the anterior part of the glabella. A suture is stated to occur at the external margin of the flat border. The thorax consists of from twenty-five to twenty-nine segments; its axis is narrow, and the pleurae are long and grooved. The pygidium is very small, and consists of three or four segments. Ordovician to Devonian. Genus: Harpes.

FIG. 147.—Paradoxides bohemicus, Barr. × ½. Middle Cambrian. (After Zittel.) ]

FIG. 148.—Holmia kjerulfi, Linnars. × 1. Lower Cambrian. (After Holm.) ]

=Fam. 5. Paradoxidae= (Figs. 147, 148, 149).—The cephalic shield is large, and bears long genal spines. The glabella is more or less swollen in front. The facial sutures appear to be absent in some genera, and when present extend from the posterior to the anterior margin. The palpebral lobes are long, and often more or less semicircular or kidney-shaped. The thorax is long, and consists of from sixteen to twenty segments with their pleurae produced into spines. The pygidium is very small, and plate-like, or sometimes in the form of a long spine. Cambrian. Genera: Olenellus, Holmia, Mesonacis, Olenelloides, Paradoxides, Zacanthoides, Centropleura (Anopolenus). Remopleurides (Fig. 150, D) from the Ordovician is usually included in the Paradoxidae, but probably belongs to a separate family.

FIG. 149.—Clenelloides armatus, Peach. Lower Cambrian, × 3. (After Peach.) ]

=Fam. 6. Conocephalidae (Conocoryphidae)= (Fig. 150, E).—The cephalic shield is semicircular, and larger than the pygidium. The glabella narrows in front. The facial suture passes from near the genal angle on the posterior border to the antero-lateral margin, and limits a large fixed cheek and a narrow free cheek. Eyes are absent or rudimentary, but an eye-line is usually present. The thorax consists of from fourteen to seventeen segments with grooved pleurae, which may be pointed, but are not usually produced into spines. The pygidium is small, and formed of few segments. Cambrian. Genera: Conocoryphe, Atops, Ctenocephalus, Bathynotus.

=Fam. 7. Olenidae= (Figs. 142, 143; 150, B, C).—The cephalic shield is larger than the pygidium. The glabella is either rectangular or parabolic. The facial suture passes from the posterior to the anterior margin. The palpebral lobes are of moderate or rather large size, and are connected by an eye-line with the front part of the glabella. The thorax includes from eleven (occasionally fewer) to eighteen segments with grooved pleurae. The pygidium is usually small, with from two to eight segments. Principally Cambrian. Genera: Ptychoparia, Angelina, Solenopleura, Sao, Agraulos (Arionellus), Ellipsocephalus, Protolenus, Olenus, Peltura, Acerocare, Eurycare, Ctenopyge, Leptoplastus, Triarthrus, Parabolina, Sphaerophthalmus, Parabolinella, Ceratopyge (position doubtful). Dikelocephalus is usually placed in the Olenidae, but perhaps belongs to a distinct family.

=Fam. 8. Calymenidae= (Figs. 136, 137).—The glabella is broadest behind. The facial suture starts at or near the genal angle—sometimes on the posterior border just inside the angle, sometimes on the lateral border just in front of the angle; the suture may be continuous with the other suture in front of the glabella, or may cut the anterior margin, beneath which it is connected with the other suture by means of a transverse suture (Fig. 137, B, D). The eyes are rather small. The thorax consists of thirteen segments with grooved pleurae; the pygidium of from six to fourteen segments. Ordovician to Devonian. Genera: Calymene, Synhomalonotus, Homalonotus.

FIG. 150.—=A=, Harpes ungula, Sternb., Ordovician. =B=, Ellipsocephalus hoffi, Scloth., Cambrian. =C=, Olenus truncatus, Brünn., Cambrian. (After Angelin.) =D=, Remopleurides radians, Barr., Ordovician. =E=, Conocoryphe sulzeri, Barr., Cambrian. =F=, Illaenus dalmanni, Volb., Ordovician. =G=, Proëtus bohemicus, Corda, Silurian, × 1½. =H=, Aeglina prisca, Barr., Ordovician, × 3. =I=, Phacops sternbergi, Barr., Devonian. (=A=, =D=, =E=, =G=, =H=, =I=, after Barrande; =B=, =F=, from Zittel; natural size except =G=, =H=.) ]

=Fam. 9. Asaphidae= (Fig. 150, F).—The body is oval and commonly rather large. The cephalic shield is large, with its glabella often indistinctly limited and the glabella-furrows often obscure. The facial suture starts from the posterior margin and usually cuts the anterior margin, but is sometimes continued in front of the glabella. The relative size of the fixed and free cheeks varies greatly. The eyes are of variable size. The thorax consists of eight or ten (sometimes fewer) segments; the pleurae are generally grooved, but sometimes plane. The pygidium is large, often being similar in form and size to the head; it consists of numerous segments which, however, may be indistinctly shown; the axis in some forms is obsolete. Upper Cambrian (Tremadoc) to Silurian; common in the Ordovician. Genera: Asaphus (sub-genera, Megalaspis, Asaphellus, Symphysurus, etc.), Ogygia, Barrandia, Niobe, Nileus, Illaenus, Bumastus, Stygina. Aeglina (Fig. 150, H) is usually placed in this family, but its systematic position is doubtful.

=Fam. 10. Bronteidae.=—The general form is similar to that of the Asaphidae. The glabella broadens rapidly in front, and is marked with furrows on each side, which are usually short, and may be indistinct. The facial suture passes from the posterior margin to the crescentic eye which is situated rather near the posterior border, and from thence to the anterior margin. There are ten thoracic segments with ridged pleurae. The pygidium is longer than the head, and has a very short axis, from which the furrows on the pleural part radiate. Ordovician to Devonian. Genus: Bronteus.

=Fam. 11. Phacopidae= (Figs. 138; 150, I; 151, C).—The head and pygidium are of about the same size. The glabella is distinctly limited, and wider in front than behind, with a neck-furrow and three other furrows, of which some of the anterior may be indistinct or obsolete. The eyes are schizochroal and usually large. The facial suture begins at the lateral margin and unites with the suture of the other side in front of the glabella. There are eleven thoracic segments with grooved pleurae. The pygidium is usually large, with a distinct axis and many segments. Ordovician to Devonian. Genera: Phacops, Trimerocephalus, Acaste, Pterygometopus, Chasmops, Dalmanites, Cryphaeus.

FIG. 151.—=A=, Phillipsia gemmulifera, Phill., Carboniferous. =B=, Arethusina konincki, Barr., Ordovician. =C=, Dalmanites limulurus, Green, Silurian. (After Hall.) =D=, Cheirurus insignis, Beyr., Silurian. =E=, Deiphon forbesi, Barr., Silurian. =F=, Acidaspis dufrenoyi, Barr., Silurian. (=A=, =B=, from Zittel; =D=, =E=, =F=, after Barrande; natural size.) ]

=Fam. 12. Cheiruridae= (Fig. 151, D, E).—The glabella is convex or inflated, and distinctly defined. The facial suture passes from the lateral to the front margin. The free cheeks are small, and the eyes usually rather small. There are from nine to eighteen (usually eleven) thoracic segments; the pleurae have ridges or grooves and free ends. The pygidium is small, consisting of from three to five segments often produced into spines. Upper Cambrian to Devonian. Genera: Cheirurus, Deiphon, Placoparia, Sphaerexochus, Amphion, Staurocephalus.

=Fam. 13. Proëtidae= (Figs. 150, G; 151, A, B).—The body is rather small, and the head forms about a third of its entire length. The glabella is sharply defined, and its furrows are sometimes indistinct; the posterior furrow curves backward to the neck-furrow, thus limiting a basal lobe on each side of the glabella. The eyes are often large (Fig. 150, G); but in Arethusina (Fig. 151, B), in which an eye-line is present, they are small. The facial sutures pass from the posterior to the anterior margin. The free cheeks are large. There are from eight to twenty-two thoracic segments with grooved pleurae. The pygidium is usually formed of numerous segments, and its margin is usually entire. Ordovician to Permian. Genera: Proëtus, Arethusina, Cyphaspis, Phillipsia, Griffithides, Brachymetopus, Dechenella.

=Fam. 14. Encrinuridae.=—The cephalic shield is ornamented with tubercles. The free cheeks are narrow, and the eyes very small. The facial suture extends from the lateral margin (or from the genal angle) to the anterior margin. There are from ten to twelve thoracic segments with ridged pleurae. On the axis of the pygidium numerous segments are seen, but usually fewer are indicated on the lateral parts. Ordovician and Silurian. Genera: Encrinurus, Cybele, Dindymene.

=Fam. 15. Acidaspidae= (Fig. 151, F).—The cephalic shield is broad, with a spinose margin, genal spines, and sometimes spines on the neck-ring. The glabella has a longitudinal furrow on each side, due to the backward bending of the lateral furrows. The facial suture passes from the posterior border (near the genal angle) to the anterior border. The free cheeks are large; the eyes small. There are from eight to ten thoracic segments with ridged pleurae, which are produced into long backwardly directed spines. The pygidium is short, and is formed of two or three segments with long spines at the margin. Ordovician to Devonian. Genus: Acidaspis.

=Fam. 16. Lichadidae.=—The body is broad, with a granular dorsal surface. The cephalic shield is small and short, with spinose genal angles. The glabella is broad, and its anterior furrows are directed backwards, limiting a convex median lobe and some lateral lobes. The facial suture extends from the posterior to the anterior margin. There are nine or ten thoracic segments with grooved pleurae, which have pointed ends. The pygidium is large and triangular, with a short axis and a toothed margin. Ordovician to Devonian. Genus: Lichas (sub-genera, Arges, Dicranogmus, Conolichas, Ceratolichas).

INTRODUCTION TO ARACHNIDA, AND XIPHOSURA

A. E. SHIPLEY, M.A., F.R.S.

Fellow of Christ’s College, Cambridge, and Reader in Zoology in the University