FOSSIL ICHTHYOLOGY; COMPRISING THE GANOID, CTENOID, AND CYCLOID FISHES.

The fishes we have hitherto examined belong to the first order, the Placoidians; we now pass to the fossil remains of the second order, the Ganoidians, which are distinguished by their brilliant angular scales, formed of osseous or horny plates, densely covered with enamel. This order contains six or more families, comprising many genera and numerous species; our investigation must be restricted to a selection from the principal genera of the Ganoids, properly so called, and of the Sauroids, or lizard-like fishes.[530]

The first family, termed Lepidoides, contains several genera, which are defined as possessing either numerous rows of brush-teeth, or of obtuse conical teeth; flat, rhomboidal scales, arranged parallel with the body; and an osseous, or partially osseous, skeleton. In one division of this family, the body is either elongated or fusiform, the mouth furnished with brash-teeth only, and the tail heterocercal, or unequally bilobed (see p. 576). To this group belong several genera, which are restricted to the Secondary formations more ancient than the Oolite; while the other group, with homocercal tails, lived in the Oolitic and Cretaceous seas. Two genera, in particular, abound in the Permian and Carboniferous strata; namely, Amblypterus and PalÆoniscus.[531]

Amblypterus. Lign. 187. Wond. p. 740. Bd. pl. xxvii.^b.—The fishes of this genus, as the name indicates, have very large and wide fins, composed of numerous rays. The scales are rhomboidal and finely enamelled; the tail is heterocercal. The figures referred to convey a correct idea of the form and external characters. Beautiful pyritous imprints of Amblypteri occur in the Carboniferous slate of SaarbrÜck, in Lorraine; and fine specimens in the ironstone nodules of the same locality. On the shore at Newhaven, near Leith, similar fossils occur in nodules washed out of the cliffs of coal-shale (Bd. p. 278).

PalÆoniscus. Ly. p. 304.—The fishes of this genus differ from those of Amblypterus in the relatively moderate size of the scales, and the numerous little rays on their margins. They have rhomboidal scales, which in some species are very small, and in others large. They have numerous brush-teeth. Several peculiar species, found in the marl-slates and magnesian limestones of the Permian system, are very widely distributed, occurring in the British Isles, Germany, and the United States.

In some localities the small species occur in groups; on a slab of red sandstone, in the Museum of the Geological Society, from Tyrone, between two and three hundred perfect fishes (P. catopterus) are imbedded on a space not exceeding two feet square.

A remarkable circumstance relating to the fishes of this genus is the almost constant absence of the bodies of the vertebrÆ in otherwise well-preserved specimens, and in which the spinal processes and the ribs are entire: occasionally, however, examples occur with some of the vertebrÆ perfect. An explanation of the above phenomenon may perhaps be found in the probable originally cartilaginous nature of the bodies of the vertebrÆ, and the osseous structure of the enduring apophyses and ribs;[532] while those rare specimens which possess a few bony vertebrÆ may be regarded as exceptions, in which ossification took place in a structure essentially cartilaginous.

The fishes found in the copper-schists of the Zechstein, at Mansfeld in Saxony, are generally impregnated with copper pyrites, and their scales are as brilliant as burnished gold. These ichthyolites are almost always in contorted and twisted positions; which appearance M. Agassiz attributes to contraction of the muscular tissues after death, during the progress of decomposition, and before the fishes sank down and became imbedded in the mud. (Poiss. Foss. tom. ii. p. 70.)

The fishes of the genus PalÆoniscus are often found in the shales and marls of the Permian and Carboniferous systems of England and Scotland. At East Thickley, in the county of Durham, numerous specimens have been found.[533] The lower Carboniferous strata at Burdie-house, a locality we have before mentioned, have yielded several species of PalÆoniscus, associated with teeth and other remains of large sauroid fishes.[534] On the continent also they prevail in deposits of the same epoch; Eisleben and Mansfeld, iii Saxony, are well-known localities. In North America they have been discovered in strata of probably the same age.[535] In fine, the genera Amblypterus and PalÆoniscus may be regarded as characteristic "medals" of the geological epoch which intervened between the Devonian and Triassic formations.

We will next examine a few genera of the homocercal Lepidoids and Pycnodonts, whose relics are chiefly distributed in the Lias, Oolite, Purbeck, and Wealden.

Dapedius. Lign. 195.—At Lyme Regis, and other productive localities of the fossils of the Lias, large masses of angular enamelled scales, and occasionally entire specimens of the fishes to which they belonged, have for many years been collected. Sir H. De la Beche first scientifically investigated the structure of these Ichthyolites, and pointed out their characters and relations. The Dapedius (of which a restored figure is given in Lign. 195) is a flat, laterally compressed fish, with a rounded head, and fins of moderate size. The body rapidly contracts towards the root of the tail, the fin of which is large and symmetrically lobed. The mouth is furnished with several rows of small conical teeth, which are crenated at their summits, and has brush-teeth on the palatine bones; the jaws are short. The scales are rhomboidal, highly polished, and united laterally by short processes; as in many other ganoid fishes. It belongs to the Lepidoids.

The Tetragonolepis is a Liassic fish, very similar in shape to the Dapedius. It was formerly grouped with the Lepidoidei, but Sir P. Egerton has lately discovered that it essentially differs from those fishes in the character of its scales and teeth, and that it belongs to the Pycnodonts.

Lepidotus.[536] Lign. 186, 196, 197.—Scales of a dark-brown, almost black colour, with a glossy enamelled surface, and of a rhomboidal or lozenge form, and teeth equally dark and glossy, of an obtuse hemispherical figure, are very common in the Wealden strata of the south-east of England and in the Isle of Purbeck. They are called by the quarry-men fishes' scales and eyes. The collectors of the last century used to term the obtuse circular teeth of this and the related genera Bufonites, from a supposition that they were formed in the heads of toads. These relics belong to an extinct genus named Lepidotus, which contains numerous species, that are distributed in the Oolite, Purbeck, and Wealden formations. These fishes resembled the Carps in their general form, but they have no anatomical relations to that family. The body is covered with large rhomboidal scales, which are protected on the external surface by a thick plate of enamel (Lign 196, fig. 3). The lateral line, which is slightly arched, passes direct from the operculum to the middle of the insertion of the caudal fin. The head, and even the face, are cased with osseous and enamelled plates. The bones of the surface of the skull are very large, and are connected by sinuous sutures. The jaws are short and rounded, and furnished with a row of obtuse, conical, circular teeth (see Lign. 197), and several rows of sessile teeth, more or less contracted at the base, which forms a very short pedicle that is anchylosed to the bone. The fossil Lepidoti are found, for the most part, in fluviatile deposits, as in the Purbeck and Wealden strata; and it is probable they inhabited the rivers or sea-coasts, and not deep waters.

The scales and teeth figured Lign. 196, 197, belong to the larger species of the Wealden. The remains of this fish were first collected in Tilgate Forest, and several teeth and: scales are figured Foss. Til. For. pl. v. and x.; considerable portions of connected scales have since been found; also the head entire, and the fins more or less perfect. A specimen in my collection retained a mass of the scales near the insertion of the tail, a foot wide; indicating the original to have been twelve feet long, and its body three feet broad. The scales are distinguished from other species by the folds or grooves on their enamelled surface; and the teeth by the contracted base, or pedicle, which is a little narrower than the crown (Lign. 197, and Pl. VI. fig. 10). A species (L. Fittoni) closely related to the above is equally abundant in the Weald of Sussex; the scales are not striated, and the teeth have no pedicle.

The intimate structure of the teeth of the Lepidotus is beautifully preserved, and may be easily examined in thin transverse and vertical sections, viewed by transmitted light: see Pl. VI. fig. 10. The dentine is composed of bundles of tubes, continued from the cells of the osseous base, radiating in a vertical direction to the surface of the tooth, as seen in Pl. VI. fig. 10, and giving off branches at an acute angle; but when more highly magnified, the finer branches are seen to be spread out and arched at their extremities, "presenting the appearance of the stems of corn, beaten down by heavy rain."[537]

The dorsal and pectoral fins of these fishes are very strong, and consist of several bony rays. There is a double row of acuminated enamelled scales, arranged more or less obliquely, on the anterior margin of the dorsal and anal fins, and on both margins of the caudal: part of the first ray of a dorsal fin, with scales, is represented Lign. 196, fig. 5.

A small species of Lepidotus (A. minor) is common in the Purbeck limestone, and specimens may often be procured from the quarries near Swanage;[538] it has also been found at Hildesheim, in Saxony, by M. Roemer. The detached scales abound in the limestones; and the splendid fossil reptile from Swanage, figured Petrif. Lign. 38, is sprinkled with the scales and minute teeth of this fish.

The majority of the species of Lepidotus belong to the Lower Oolites and the Lias. The habits of the Lepidoti, as indicated by the form and structure of the teeth, were those of fishes whose food consisted of crustaceans, shelly mollusca, &c.; for the dental organs are peculiarly adapted for the crushing and grinding of such substances; and the teeth of the adult fishes are generally worn down by use.

Pycnodus. Pl. I. fig. 3; Lign. 194, fig. 3.—The fishes of the family of Pycnodonts, so named from the thickness of their teeth, have an osseous skeleton, a flat body covered with rhomboidal scales, and flat or rounded teeth disposed in several rows on the palatine, vomerine, intermaxillary, and premandibular bones.[539] As in the Lepidotus, these teeth are constructed for crushing, and have generally a smooth, dense, convex or flattened crown, with a highly polished surface. This genus belongs chiefly to the Oolite; it is found also in the Lias, Chalk, and Wealden. A perfect fish of the genus Pycnodus (P. rhombus), from the Jura limestone, at Torre d’Orlando, near Castellamare, is figured in the frontispiece of the first volume of this work; and a vomerine bone, with teeth, from Tilgate Forest, in Lign. 194, fig. 3. In the last fossil there is a median row of flat,, elongated, transversely arched, smooth, glossy teeth, with a double alternate row of small sub-circular teeth on each side, attached to the bone, which is imbedded in Tilgate grit. Specimens of this kind, belonging to one or more species of Pycnodus, occur in the Wealden of Sussex; they were among my earliest discoveries in Tilgate Forest (Foss. Til. For. pl. xvii. figs. 26, 27). Examples are met; with in which all the teeth are shed, and the bony plate of the vomer alone remains.

Gyrodus. Lign. 198.—In another genus of the Pycnodonts, termed Gyrodus, the crowns of the teeth are deeply furrowed, the structure of the dentine is very dense, and the pulp-cavity large and simple. One species occurs in the Speeton clay of Yorkshire, and another in the Sussex weald; but the teeth are chiefly found in the Oolite and Chalk. As in Pycnodus, the teeth are distributed in rows on the bones composing the roof, floor, and sides of the mouth.[540]

These characters are beautifully displayed in the Russian specimen, Lign. 198. This interesting fossil was presented to me by the late Stephen Cattley, jun. Esq., who collected it in 1839, in a valley near Rjeff, a village on the banks of the Volga. Mr. Cattley informed me "that many fossils are found in that and the neighbouring valleys; and the locality is frequented by Russian geologists when the season permits, which is but seldom, owing to the long duration of the snow, and the heavy rains which accompany the thaw." This specimen consists of the vomerine bone, which is of a coarse texture, and five rows of teeth; the median row consists of very large elliptical teeth; those of the lateral rows are much smaller and arranged alternately. The peculiar structure of the teeth of this genus[541] is finely displayed in this fossil. The ample, deep, and simple pulp-cavity is seen in several teeth, where the crown of dentine has been worn off, filled with a pure white calcareous spar; one of these cavities is marked a. The dentine is extremely dense, consisting of very minute calcigerous tubes, and passes into an external layer of enamel.

The fishes of the genus Gyrodus have the body large, flat, and elevated; the dorsal and anal fins are very long; and the tail is forked, with equal elongated lobes. The scales are laterally connected by strong processes, as in Lepidotus.

Other genera related to the foregoing occur in the Oolite; as for example, Microdon, thus named from the smallness of its very numerous flat angular teeth, arranged in many rows; Placodus, in which the teeth are few, flat, and of great size;[543] and Platysomus (flat-body), with orbicular, clavate, teeth.

In these fishes, also, the dental organs are well adapted for the comminution of shell-fish, and other hard bodies.

Cephalaspides of the Devonian System.—The remains of the three genera of Ganoid fishes that we have now to notice are of a very remarkable character, and are found exclusively in the Devonian or Old Red system; most frequently in Scotland, but also in other parts of the British Isles, and in Europe and America. These fishes agree in one general character, that of having extensive osseous plates, or scutcheons; their general aspect will be understood by reference to Lign. 199, 200, 201. There are no vestiges of the bodies of the vertebrÆ, which, therefore, were probably cartilaginous. These fishes constitute a distinct family with the name Cephalaspides, from the character of the first genus we propose to describe.

Cephalaspis Lyellii. Lign. 199, 200.—The most striking feature in the Ichthyolites of this genus is the enormous scutcheon, or buckler, which forms the head, and is prolonged posteriorly into two lateral horns or points; this part so closely resembles the cephalic shield of certain trilobites (see Lign. 175), that the first found specimens were supposed to be the remains of unknown crustaceans. The name Cephalaspis (buckler-head) is derived from this character. This remarkable appearance is occasioned by the intimate anchylosis of all the bones of the cranium. The body of these fishes is relatively smaller than the head; it has one dorsal fin, and terminates in a tapering tail, supporting a fin. There are two small eyes, placed towards the middle of the head. The body is covered with rhomboidal scales; and the head with discoidal scales, which are highly ornamented with radiated markings[544] (Ly. p. 344, fig. 396). There are four species of Cephalaspis at present known.

The other genera are equally unlike any recent types of the class of fishes. No perfect examples have been found, and some parts of their structure are still unknown; the annexed figures, Lign. 201, have been drawn by Mr. Dinkel (the eminent artist employed by M. Agassiz), with scrupulous accuracy, no part being introduced which is not clearly demonstrated in some one specimen; and the form of the scutcheons is made out very distinctly, to aid the collector in discriminating the detached plates, which are the most common relics of these singular beings.

Pterichthys cornutus.[545] Lign. 201, fig. 1.—This fish is distinguished by its two wing-like lateral appendages; whence the name of the genus (winged-fish). These processes, like the spines on the gill-covers of the common Bullhead (Cottus gobio), are weapons of defence. In some specimens they are extended at right angles to the body (Ly. p. 345, fig. 400).

The head and anterior part of the body are covered with large angular tuberculated scutcheons. The under surface of the body is flat and protected by five plates, a quadrangular plate occupying the centre. The upper part of the body is convex; the form and disposition of the dorsal scutcheons are shown in Lign. 201, fig. 1. There are two eyes, which are placed in front of the lateral spines: see fig. 1. The tail is of an angular form, and as long as the body; it is covered with scales, and considered by M. Agassiz to have been the only instrument of locomotion. The British species of this genus, of which ten are known, are all very small, varying in length from one to ten inches.

Coccosteus oblongus. Lign. 201, fig. 2.—The fishes of this genus, as may be seen by the lignograph, very much resemble those of Pterichthys; in both the osseous scutcheons of the body are very similarly disposed. In Coccosteus, the head[546] is somewhat rounded; slight rounded notches on the edge of the buckler indicate the place of the eyes on the sides of the head. There is no indication of lateral spines. The tail is very long, covered with scales, and supports a fin. The plates of the body are tuberculated, as in Pterichthys. There are six or eight teeth on each half of the lower jaw (and probably as many on the upper), with a curious group of teeth situated on its symphisis. The teeth are chiefly composed of bone, passing into dentine at their surface.[547]

These fishes are from a few inches to two feet in length; six species have been discovered; and their remains are the most abundant of the Ichthyolites of the Old Red. Patches of detached scales, and isolated osseous plates, are very frequent in the sandy cornstones, and the subcrystalline masses of limestone. These fragments are usually of a brilliant blue or purple colour; and, strongly contrasting with the dull red tint of the surrounding rock, are easily recognised. The colour is supposed to be due to the presence of phosphate of iron, which has communicated a similar tint to the Ichthyolites of the Caithness Schist.[548]

Fossil Sauroid Fishes. (Poiss. Foss. tom. ii.)—The family of Ganoid, fishes termed Sauroid, or lizard-like, by M. Agassiz, are so named in consequence of certain peculiarities in their organization which are found in no other animals of this class, but exist in reptiles. There are but two living genera; namely, the Lepidosteus, of which there are many species that inhabit the rivers of America; and the Polypterus, that contains two species, one inhabiting the Nile, and the other the rivers of Senegal. In these fishes the bones of the skull are closely connected by sutures; the teeth are large, conical, and longitudinally striated, as in the crocodile, plesiosaurus, &c.; the spinous processes are united to the bodies of the vertebrÆ by suture, as in most reptiles; and the ribs are articulated to the extremities of the transverse processes; the skeleton is osseous. Even in the soft parts many analogies to reptilian structure are seen; thus the Lepidosteus has a glottis, as in the Siren; and a cellular air-bladder, with a tracheal vessel, resembling the lungs of an Ophidian (serpent). These fishes are the only living representatives of those voracious tribes of the most ancient marine fauna, whose remains abound in the Secondary formations. Their relics have often been mistaken for those of reptiles; particularly the teeth, which from their large size, conical figure, enamelled and striated surface, and internal cavity, were generally supposed to belong to crocodiles. The scales are flat, rhomboidal, and parallel to the body. The recent Lepidosteus osseus, of North America, affords a good illustration of the fossil genera; a reduced figure of this fish, from Poiss. Foss., will be found in Bd. pl. xxvii^a.; and teeth of some fossil Sauroids are represented Bd. pl. xxvii.

The teeth of the Sauroids consist of two kinds; namely, large pointed striated cones, and numerous small brush-teeth. The intricate structure of the conical teeth of the Stony-gar, or Lepidosteus, is very remarkable, and presents some analogy to that observable in the dental organs of the Labyrinthodon, an extinct genus of reptiles, of which we shall treat in a subsequent chapter. The tooth consists of a large conical pulp-cavity, surrounded by a mass of dentine, which is plicated longitudinally, its folds giving to the pulp-cavity an appearance of being divided into parallel longitudinal branches; resembling, in this respect, the base of the tooth of Ichthyosaurus, as shown in a transverse section, Pl. VI. fig. 9. If we imagine these folds to be multiplied, and to have more inflections, and the pulp-cavity to be reduced in its proportions, we shall have the elegant organization of the teeth of the Labyrinthodonts (see Pl. VI. fig. 3b). The dentine is composed of very minute calcigerous tubes, which pass off at right angles from the pulp-cavity to the periphery; and it is covered by a layer of cement, or coarser dentine, which is encased in a coat of enamel, forming the external investment of the tooth.[549] The long conical teeth are implanted in alveoli or sockets, to the walls of which they are anchylosed at their base.

Lepidostei.—This family, having the above described recent Lepidosteus for its type, comprises several genera, and is represented in all the formations from the Tertiary to the Coal-measures inclusive. The Lepidosteus itself has left its remains in the Eocene tertiary of England. The Lepidotus (before described, p. 604,) ranges from the Chalk to the Lias; the Pholidophorus and Dapedius (p. 603) abound in the Lias; the latter being found also in the Wealden, and the former in the Oolite and Purbeck; and the PalÆoniscus (see p. 601) is a well-known Permian and Carboniferous form.

Sauroidei.—The remains of the Sauroids proper occur in the Chalk, Purbeck, Oolite, Lias, Permian, and Carboniferous rocks. The great strength and size of some of these teeth prove that the seas of those remote periods were inhabited by voracious fishes of enormous magnitude. See Petrif. pp. 432, et seq.

The teeth and jaw of a gigantic sauroid (Megalichthys), from the Carboniferous strata at Burdie-house, are figured Bd. pl. xxvii.; the sections of the teeth shown in figs. 13, 14, of that plate, illustrate the size of the pulp-cavity and the thickness of the layer of dentine. These remains were associated with the plants and crustaceans previously described; an assemblage of fossils indicating a lacustrine or estuary formation. Similar teeth have been found in the cannel-coal of Fifeshire.

In the Upper Lias of Ilminster, Somerset, the nodular calcareous rock is rich with the remains of the Pachycormus, many fine specimens of which Mr. C. Moore, of that town, has successfully worked out from their stony encasement, exhibiting the gaping, contorted fish, as it died in the convulsive throes of suffocation in the muddy sea. The little sauroid Leptolepis, of the Lias and the Purbeck, is also abundant in the above mentioned locality.

In the lithographic stone of Solenhofen, and in the Purbeck strata, Oxford Clay, and Lias of England, occur specimens of a sauroid fish remarkable for the extreme shortness of the lower jaw, and the prolongation of the upper into a beak; it is named Aspidorhynchus by Agassiz. The figure of this fossil fish is contrasted on pl. xxvii^a. Bd. with that of its recent ally, the Lepidosteus.

In the Shanklin sand and in the Galt of Kent and Sussex, large, conical, striated teeth belonging to sauroid fishes are occasionally found. They invariably occur detached, and no portion of the jaws has been observed. In the Chalk of Sussex several fine sauroid fishes have been discovered; such as the Lophiostomus,[550] Belonostomus,[551] and Caturus; the last two of which are found in the Oolite and Lias also. These have been described and figured in the late Mr. Dixon’s "Geology and Fossils of Sussex."

The Sauroidei-dipterini[552] are found almost exclusively in the Devonian formation. Osteolepis, Diplopterus, and Dipterus are characteristic members of this family. Interesting descriptions and instructive figures of the structure of these genera are given in Mr. Hugh Miller’s late work, "Foot-prints of the Creator, or the Asterolepis of Stromness," at pp. 50 to 61.

Coelacanthi.—This is a numerous family of sauroid fishes, that have derived their name, hollow-spine, from the central cavity in the fin-rays, which, however, may have had originally cartilaginous cores. They are found from the Devonian to the Cretaceous series. In the former, one of the most remarkable is the genus Holoptychius, distinguished by the peculiar structure of the scales; the enamelled surface of which is covered by undulated furrows. The whole body is covered by thick enamelled scales of this kind. A splendid specimen, twenty-eight inches long, and twelve wide, is figured Murch. Sil. Syst. pl. ii.^bis. Scales have been found exceeding three inches in length, by two and a half in width; which must have belonged to a fish of great magnitude. Ly. fig. 395.

In the Old Red Sandstone of Elgin, at a quarry at Scat-craig, some peculiar teeth occur, which possess a very remarkable structure, and have been referred to a genus of sauroid fishes, which, from the dendritical or arborescent disposition of the calcigerous tubes, Prof. Owen has named Dendrodus. These teeth are of a conical form, slightly curved, and solid throughout. On the external surface they are finely striated longitudinally, and have two opposite vertical ridges; the base is rough, and its margin rounded, as if for attachment to a shallow socket. The largest specimen is one and a half inch in length. In thin sections, viewed microscopically by transmitted light, there is a central pulp-cavity, of small size; the medullary canals pass into a few short ramifications, like the branches of a shrub, and these are distributed into irregular dilatations, simulating leaves, which resolve themselves into radiating bundles of calcigerous tubes; a portion of a transverse section[553] is shown Pl. VI. fig. 8.

In Mr. H. Miller’s charming work just referred to we have a full and clear account of the singular fossil fish the Asterolepis. Remains of this gigantic Ganoid were first found in Russia.[554] Its name is derived from the stellate markings on the dermal plates of the head, which are of great size, and form a strong expanded buckler, the orbits of the eyes being situated near the anterior border. (See Miller’s Asterolepis, pp. 74, et seq. figs. 27-29.) Lign. 202 shows a part of the lower jaw of this fish, seen on the inside. Along the upper margin are seen a "thickly set row of small broadly-based teeth,"—these are ordinary fish-teeth; and behind this edge-row of small teeth, b, there occur "a thinly set row of huge reptile-teeth, based on an interior platform of bone, which formed the top of the cartilage enclosing box composing the jaw." (Miller.) These large teeth, a a, are longitudinally striated, and generally bear two sharp lateral cutting edges.[555]

Macropoma Mantelli. Wond. p. 348. (Foss. South D. tab. xxxvii. xxxviii.; Petrif. p. 436, Lign. 89.)—This Coelacanth fish is from one to two feet in length, of an elongated fusiform shape, with a large head and two dorsal fins; the anterior fin is imbedded, and has seven or eight strong spinous rays, the first two of which have numerous spines. The opercula are very long and large (hence its name); the scales[556] are garnished with adpressed spines, disposed in semicircular rows (see Lign. 185, fig. 2). The teeth are small, conical, and numerous.

Several specimens of this fish are almost perfect. The bones of the cranium, the jaws, teeth, opercula, branchial rays, palatine arches, the surface of the body covered with scales, all the fins, the pelvic bones, the vertebrÆ and their apophyses, all remain.

In one example, the vomer, covered with minute teeth, is exposed. But the most extraordinary fact relating to these Ichthyolites, is the preservation, in every specimen, of the air-bladder;[557] even its membranes remain, and separate in flakes; and the ramification of the minute vessels is visible under a high magnifying power. In some instances this bladder is displaced and much distended; but in general it occupies its natural position, and retains its elongated, sub-cylindrical form, with a few annular constrictions or folds.

Coprolites. Lign. 139, figs. 1, 2.—In more than one example the solid earthy residue of digestion, in small lumps or pellets, of a conical form and spirally convoluted, lies in the abdomen of these fossil fishes. Fossil excrementitious substances of this kind are termed Coprolites (Bd. p. 198, pl. xv.); they occur in many deposits, and belong not only to fishes, but to large reptiles and other animals. Lign. 139, p. 432, fig. 1, represents the coprolite of a Macropoma; and fig. 2, that of a species of Shark, from Hamsey. The convoluted appearance of these bodies arises from the peculiar organization of the intestinal canal of the original fishes, in which, as in the recent Dog-fish, a portion of the intestine was spirally twisted, the tube forming several gyrations; and the passage of the calcareous substance through this constricted canal gave rise to the structure observable in the coprolites. In the Macropoma the gyrations appear to have been few; seldom more than five or six turns being apparent. In the fossil Sharks the convolutions are more numerous, ten or twelve occurring in the length of an inch. In many of the coprolites, the impression of the mucous or lining membrane of the intestinal canal may be detected. Some of the coprolites of the Macropoma are not convoluted; probably from having occupied the upper part of the digestive tube, and therefore not having been moulded in the spiral part. (See Geol. S. E. p. 145. Foss. South D. tab. ix.) Minute scales and bones of fishes are occasionally imbedded in the substance of the coprolites, affording evidence of the carnivorous habits of the Macropoma.

Cololites. Bd. pl. xv^a—It will be convenient to notice in this place those curious fossils which occur in detached masses in the limestone of Solenhofen, and used to be known to collectors by the term lumbricaria, from the supposition that they were petrified earth-worms. An excellent representation of a fine specimen is given by Dr. Buckland in the plate referred to above. These convoluted bodies M. Agassiz, with his wonted sagacity, has ascertained to be the intestines of fishes; and has therefore named them Cololites. Although generally found isolated, specimens occur in which they are imbedded, like the coprolites of the Macropoma, in the abdominal region of fishes. The frequent occurrence of the Cololites apart from the body, is explained by the usual process of decomposition in fishes. M. Agassiz remarks that dead fish always float on the surface of the water with the belly uppermost, until the abdomen bursts from distention. The small intestines are then expelled by the evolved gases through the aperture, and soon become detached from the body. I have observed indistinct traces of similar remains in the beds of chalk in which fossil fishes most prevail. Dr. Buckland mentions the discovery, by Lord Greenock, of a mass of petrified intestines distended with coprolite, and surrounded by the scales of a fish, in a block of coal-shale from the neighbourhood of Edinburgh (Bd. p. 199).

Dercetis elongatus. Wond. p. 349.—Before proceeding to the investigation of examples of the next order, I will describe a highly interesting Ichthyolite, which in a mutilated state is extremely common in some of the chalk strata of the South-east of England; it is noticed in Foss. South D. p. 232. This fish is placed by M. Agassiz in his family of ganoidians, termed Scleroderms; and he mentions that another species has been found in the chalk of Westphalia. The Dercetis has a very elongated body, with a short head terminating in a pointed beak; the upper jaw is a little longer than the lower, and both jaws are armed with long, conical, elevated teeth, and several rows of very small ones. On each side of the fish there are three rows of osseous scutcheons like those of the Sturgeon (see Wond. p. 349); the body was also covered with numerous small scales. From the form of the body somewhat resembling that of the eel, being very long, and sub-cylindrical in uncompressed examples, the specimens are generally called "petrified eels" by the quarry-men. The examples usually found consist of the elongated body, more or less compressed, and irregularly covered with patches of scales confusedly intermingled; among which traces of the scutcheons may sometimes be distinguished. These specimens occasionally exceed two feet in length, by one or two inches in breadth; with neither extremity perfect, and without any vestige of the fins.[558] The example figured Wond. Lign. 74, is the only instance in which I have seen the cranium perfect. The scutcheons in the Westphalian species have a prominent longitudinal ridge or keel, and their surface finely granulated; they are so large that the whole body of the fish is covered with them.

Fossil Ctenoid Fishes (Poiss. Foss. tom. iv.).—The fishes of this order have imbricated laminated scales, the posterior margins of which are round and finely pectinated; i. e. divided into little teeth, like a comb. These scales are nearly circular, but more or less elongated; and, as the laminÆ of which they are composed successively diminish from the lowermost to the uppermost, the pectinated margin of each being apparent, the surface is very scabrous; the front edge is sinuous. The common Perch is the type of the Ctenoidians. The teeth of these fishes are invariably small, and either villous or brush-like.

From the numerous fossil genera I select, in illustration of the characters of this order, the Beryx; of which four species occur in the English Chalk, and three others in the Chalk of Bohemia and Westphalia. Of this genus, which is closely related to the Perch (Perca), two living species inhabit the seas of Australia. The bones of the skull have dentated crests; the dorsal fin has spinous rays in front, which are united to the soft rays; the margin of the caudal fin has little spinous rays.

The Beryx is one of the most ancient representatives of the Perch tribe, and of the Ctenoid order.

Beryx Lewesiensis.[559] Wond. p. 351; Petrif. Lign. 90.—This is one of the most common of the Ichthyolites of the Chalk of the South-east of England; it is called "Johnny Dory" by the quarry-men; the specimens are from six to twelve inches long. It occurs also in the Chalk of Westphalia. The outline of the perfect form of this species, Wond. p. 351, by Mr. Dinkel, conveys an accurate idea of its external characters. It has one dorsal fin, with several spinous rays in front of the soft ray. The head is very large, and the opercular pieces are ornamented with sculptured rays; the margins of the jaws are covered with a broad band of brush-teeth. The orbit is large, and often contains the capsule (sclerotica) of the eye. The rays of the gills are short and thick, five are preserved in some examples. The scales are very large; about twenty-five in the median row; their posterior margins have several concentric rows of spines (see Lign. 185, fig. 3, p. 567). The lateral line is often distinctly apparent, in the form of a tube, contracted behind and expanded in the centre of the scale. The vertebral column is composed of large short vertebrÆ, with very long apophyses; the ribs are slight.

Beryx superbus.—This is a larger species, sometimes thirteen inches long, with very large and broad scales. It has been found in the lower chalk at Lewes. See Dixon’s Foss. Sussex, tab. xxxvi. fig. 5.

Beryx radians. Wond. p. 350.—This species is smaller, and relatively longer, than B. Lewesiensis; it generally occurs in the Chalk-marl, and is invariably of a very dark colour, the scales having a polished or glossy aspect. The scales are small, with a simple row of diverging spines on the posterior edge. The scales of the lateral line are peculiar; the mucous canal is not formed of a series of simple tubular cylinders, as in B. Lewesiensis, but is divided into several branches, as may be seen with a lens of moderate power. There are more than thirty scales in the length of the lateral line.

Beryx microcephalus (Poiss. Foss. tom. iv. tab. iv^c.; and Dixon, Foss. Sussex, tab. xxiv. fig. 3).—This fish is distinguished by its slender form, and the extreme smallness of the head, as the name implies. The scales have one row of very thick spines on the posterior margin; they are more elevated, and shorter than in B. radians; those of the lateral line are pierced by an elongated conical tube, and are not ramified as in the last species. This ichthyolite, like the B. radians, occurs in the Chalk-marl, and in a similar state of mineralization.

Smerdis minutus. Lign. 203.—A pretty ctenoidian fish, from one to three inches long, about the dimensions of a perch a year old, is very common in the marls of Aix in Provence, Wond. p. 260; and many are often found grouped together in every variety of position. This species is characterised by the elevated anterior rays of the dorsal, and the wide and very forked caudal fin.

Several ctenoidian and cycloidian fishes have been found in the north of the Brazils, by Mr. Gardiner, in strata probably of the Cretaceous epoch.

Fossil Cycloid Fishes. (Poiss. Foss. tom. v.)—This order comprises the fishes possessing scales of a cycloid, or circular, form, with smooth margins, and composed of plates of horn or bone, without enamel. It contains numerous families, including the Scaroids, or Parrot-fishes, and the Scomberoids, or Mackerel tribe, which are Acanthopterygians, the Lucioids, or Pikes, Clupeoids, or Herrings, Salmonoids, or Salmon tribe, and the Cyprinoids, or Carps, which are Malacopterygians, as well as other families. The fossil remains of this order are exceedingly numerous, particularly in the Tertiary and upper Secondary deposits. A genus of SalmonidÆ, discovered in the White Chalk of Sussex, will serve to exemplify the characters of the fossil cycloidian fishes.

Osmeroides. Plate II. and Wond. p. 344, 347.—Two species of this genus occur in the Chalk, near Lewes; and principally, if not exclusively, in the Lower Chalk, without flints. They are exceedingly beautiful Ichthyolites, and are almost invariably found with the body but little compressed; the fish, in many examples, is as round and perfect as when living. The entire cranium, the opercula and branchial rays, and all the fins are preserved in some examples. These fishes belong to the Salmon family,[560] and are nearly related to the Smelt (Osmerus); whence the name of the genus. There are two species, easily distinguishable. The first (O. Mantelli, Pl. II.) has a short, sub-cylindrical body, and seldom exceeds eight or nine inches in length; the other (O. Lewesiensis) has an elongated and elliptical body, and sometimes attains a length of fourteen inches. The dorsal fin too in this species has more rays than in the other. The fossil figured in Pl. II.. is a very remarkable specimen of the first species. It is nine inches in length; and the chalk has been cleared away, so as to expose the entire fish, six inches in relief above the surface of the block, Petrif. Lign. 92, p. 445. The fish is lying on its back, with the mouth open, and the opercula, or gill-covers, and the branchial arches expanded; the pectoral and ventral fins, and the dorsal fin, are in their natural position; the five rays of the dorsal are erect; of the caudal fin, or tail, but slight indications remain. There is but one dorsal fin; but in a specimen of O. Lewesiensis there is a trace of the little adipose process observable between the dorsal fin and the tail, as in the recent species of SalmonidÆ. A magnified view of one of the scales is represented Lign. 185, fig. 4, p. 567.

Of the Cyprinoids, or fishes of the Carp family (Malacopterygians), the recent species of which are inhabitants either of fresh-water, or the brackish waters of the mouths of rivers, many fossil species occur in the fluviatile and lacustrine deposits of the Tertiary formations. In their character of omnivorous fishes, the Carps then, as now, formed the principal mass of the finny population of the lakes, and in their turn served as food to the carnivorous tribes, as the pikes, eels, &c. Several species are found in a beautiful state in the schists of Œningen, and in the Tertiary marls at Aix. Many of the layers of marl at the latter locality are covered with groups of fishes of the family Cyprinodonts, the recent species of which are of a small size, and inhabit the fresh-water lakes of temperate zones. Lign. 184, p. 562, represents a portion of a large slab of marl in the cabinet of Sir R. Murchison, which is covered with scores of a species resembling a recent fish (Lebias) in the profile of its head, and the form of its fins. It is named Lebias cephalotes, from the relative largeness of its head. The black appearance of the abdomen in many of these Ichthyolites indicates the original situation of the intestines and of the liver, which is largely developed in the fishes of this family, and contains much colouring matter.

Saurocephalus and Saurodon. Lign. 204.—In the same quarry, near Lewes, from which the first entire fish of the Sussex Chalk was obtained, teeth of a very peculiar character were, many years since, occasionally discovered.[561] These teeth are of a lanceolate form, much compressed; with entire, sharp edges, terminating in a point; the fang is single, and broad; the surface of the crown is glossy, and marked with fissures filled with chalk (see Lign. 204, fig. 1). Teeth of this kind, attached to portions of the jaw, were subsequently found in the Chalk at Brighton and Lewes. Similar remains were collected from the Cretaceous marls of Missouri and New Jersey, in the United States. The American specimens comprised two closely allied genera, which, from the supposition that the fossils were the relics of reptiles, were respectively designated Saurocephalus and Saurodon.[562] Examples of the teeth and jaws of both genera have been discovered in the Sussex Chalk (see Lign. 204).

M. Agassiz retains the names imposed by the American naturalists, and has placed these genera in the family of Scomberoids (Mackerel, Swordfish). They belong to the Pharyngognathi of Muller. The teeth are disposed in a single row, and fixed in deep sockets by a simple root, or fang, which is frequently somewhat excavated by the pressure of a successional tooth. In Saurodon Leanus the crown of the tooth is angular, and barbed, and supported on a sub-cylindrical shank, or stem (see Lign. 204, fig. 3). The microscopical structure of these teeth presents that peculiar reticulated disposition of the medullary canals throughout the entire body of the tooth, which is only found in the dental organs of fishes.

Hypsodon Lewesiensis. (Foss. South D. tab. xlii.)—The Sussex and Kentish Chalk also contain the remains of a very large fish, belonging to the Pharyngognathi, with extremely upright, long, conical, compressed, pointed teeth, which, like those of the Saurodon, are implanted in sockets. These teeth are commonly of a delicate fawn colour externally and of a dark brown internally; having a large simple pulp-cavity. In Foss. South D. (tab. xlii.) are represented portions of an intermaxillary and jaw-bone with teeth; a vertebra, deeply biconcave; and a large bone, apparently a branch of the os hyoides; all found in the same block of chalk.[563]

[563] A magnificent specimen (now in the British Museum) displays, on the same slab of chalk, a large portion of the cranium, teeth, several vertebrÆ, ribs, and many other bones, belonging to a fish of considerable magnitude. Petrif. p. 444.