CHAPTER XV.

Lign. 191. Edaphodon leptognathus.1/2nat.London Clay.Bracklesham.The Upper and Lower Mandibles of the left side; viewed mesially or from within.Fig.1.—The Upper Jaw; left ramus.p m.—The premaxillary bone.1, 2, 3.—The three dental tubercles.2.—The Lower Jaw.s.—The symphysial surface, by which this ramus is unitedto the opposite or right side of the jaw.4, 5, 6.—The three dental tubercles, or triturating surfaces,of the left ramus of the lower jaw.

s.—The symphysial surface, by which this ramus is unitedto the opposite or right side of the jaw.4, 5, 6.—The three dental tubercles, or triturating surfaces,of the left ramus of the lower jaw.

s.—The symphysial surface, by which this ramus is unitedto the opposite or right side of the jaw.

4, 5, 6.—The three dental tubercles, or triturating surfaces,of the left ramus of the lower jaw.

Many specimens, both of the upper and lower mandibles, have since been discovered in the Eocene beds, Chalk, Upper Greensand, Galt, Kimmeridge Clay, and Stonesfield Slate. The subject has been carefully investigated by Sir P. Egerton; and this eminent Ichthyologist has tabulated the principal forms, and arranged them under five genera.[524]

[524]Viz. Ganodus, Ischyodus, Edaphodon, Elasmodus, and Psaliodus. See Quart. Journ. Geol. Soc. vol. iii. p. 35; and Dixon’s Foss. Sussex.

In some species the external vertical wall of the plate is formed of hard dentine, resembling enamel; in others the dentine is disposed in isolated ramifications, producing a dendritical appearance; the modifications of this structure occasion the differences observable in the dental plates of the various species. In some, compact dentine with parallel canals constitutes the mass of the tooth; in others, the squamous dentine with ramifying tubes prevails.

I have figured the right upper and lower mandibles of the type namedEdaphodon(pavement-tooth), in which there arethreeteeth or dental tubercles on each ramus of both jaws,Lign.191: the lower mandible is produced anteriorly into a falciform beak:[525]the articulating surface of the symphysis (2s) is broad at the base, and contracts gradually forward until the margins meet at the apex. InIschyodusthe lower jaw is deeper, less produced in front, and the margins of the symphysis are parallel until abruptly truncated at the extremities: the upper jaw hasfourtubercles on each side.

[525]Hence M. Agassiz proposed the name ofPsittacodon(parrot-tooth) for this genus of Chimæroids.

The upper jaw inElasmodushas but three tubercles, as in Edaphodon, but the dentine of which they are composed is confluent, being rolled round like a scroll in the substanceof the bone, one edge forming the margin of the tooth, and the other being buried deep in its centre.

The dorsal fin-ray or spine of the Edaphodon is laterally compressed, with the posterior margin grooved, and the edges armed with fine teeth: I have a specimen of the spine, with a pair of inferior mandibles of the same individual, imbedded in a block of chalk from Kent; by favour of Mrs. Smith, of Tunbridge Wells.

Lign. 192. Fossil Teeth of Sharks.Fig.1.—Hybodus medius.Lias.Lyme Regis.2.—Hybodus raricostatus.Lias.Bristol.3.—Carcharodon productus.Tert.Malta.4.—Hemipristis serra(fragment).Chalk.Ratisbon.5.—Otodus Obliquus.Tert.Sheppey.

Hybodus.Lign.192, figs. 1, 2. (Bd.pl. xxviid.)—Intermediate between the obtuse crushing teeth of the Sharks previously described, and those sharp, angular, pointed, dental organs of the Squaloids, are those of the fishes which M. Agassiz has arranged in a sub-family or group termedHybodonts; the teeth of which are characterised by their transversely elongated form, and the series of subacute, compressed, conical cusps or points, which compose the crown. The median cone is the principal, the lateralpoints being shorter and smaller, as inLign.192,fig.2; in some species the difference between the median and lateral cones is greater, in others less, as infig.1. These cusps have a coating of dense enamel, which is plicated longitudinally on both faces. The base, which almost equals the crown in size, is composed of a coarse osseous substance. The internal structure of the crown differs from that of the Cestracionts, in having no principal pulp-cavity, and in being chiefly composed of dendritical dentine, with reticulated medullary canals. The form and organization of these teeth show them to have been instruments for cutting and tearing food. The Hybodonts, as we have already stated (p. 581), possessed two spinous dorsal fins; in their habits and economy they probably did not differ from the ordinary Sharks. Teeth and spines of this genus are common in the Trias, Lias, Oolite, and Walden, and occur in the Green Sand and Chalk. There are several species of teeth and fins in the strata of Tilgate Forest (Foss. Til. For.pl. x.). In general the teeth are found detached, but occasionally they occur in their natural position, adhering to the mineralized cartilaginous jaws (Petrif.Lign.97); as in the beautiful fossil figuredBd.pl. xxvii.d; and in the fine specimen ofH. basanus, from the Isle of Wight, figured in theGeol. Soc. Journ.vol. pl. iv. There are several related genera, founded on the situation, form, and division of the principal cusps of the teeth.

Sharks with Cutting Teeth.—The jaws of the common squaloid fishes, as theLamna(Porbeagle) andCarcharias(Great White Shark), are so common in collections of natural history, as to render a description unnecessary. The numerous vertical rows of angular, laterally compressed, pointed teeth, with sharp or serrated edges—in some species consisting of a simple trenchant cusp, in others with small lateral teeth, or denticles, at the base, are characters with which all are familiar. Fossil teeth of this form are extremelyabundant in the Tertiary and Cretaceous deposits; and are commonly in a beautiful state of preservation. The genera of these fossil teeth are founded on the solidity or hollow structure of the cusps, their possessing cutting or serrated edges, and the presence or absence of lateral denticles. But the last character cannot in every instance be relied upon, for some recent Sharks have rows of teeth both with and without denticles.

Carcharodon productus.Lign.192, fig 3.—The genus Carcharias comprises the large Sharks with cutting triangular teeth, crenated (notched) on their margins, and having a broad base. InCarcharodon, the teeth differ from those of Carcharias in being solid in the centre, while in the latter they are hollow; but in both genera the teeth exhibit the same reticulated structure of medullary and calcigerous tubes. TheWhite Sharkand other large species belong to these genera; some of which are upwards of forty feet in length. But even these colossal fishes must have been far surpassed in magnitude by the extinct species of the Tertiary deposits, if the teeth afford a scale of proportions; for some of the fossil teeth from Malta and the United States are six inches long, and five wide at the base;[526]being twice the size of the teeth in the largest living species. The specimen figured in illustration,Lign.192,fig.3, is of a small size.

[526]For instance, some of the Maryland specimens ofCarcharodon megalodon. See an admirable memoir on the Fossil Squalidæ of the United States, by Dr. R. W. Gibbes, Journ. Acad. Nat. Science, Philadelphia. At the meeting of the British Association in 1851, J. S. Bowerbank, Esq. F.R.S. read some interesting observations on the comparison of these large fossil fishes with the recentCarcharias glaucusof Australia. See also Owen, Cyclop. Anat.Art.Teeth.

Hemipristis serra.Lign.192, fig 4.—The fossil teeth of this genus are distinguished by serrated edges, that do not extend to the summit, which is a sharp angular point; as in the fossil represented.

Lign. 193. Fossil Teeth of Sharks.Chalk.Lewes.Fig.1.—Corax pristodontus.2.—Lamna crassidens.3.—Notidanus microdon.4.—Ptychodus polygurus; seen laterally.5.—Ptychodus polygurus; viewed from above.6.—Lamna elegans.

Lamna elegans.Lign.193,fig.6.—The fishes of the genusLamna(to which the recent shark called thePorbeaglebelongs) have teeth with smooth trenchant edges, and a small sharp denticle (little tooth) on each side the base, as in the fossil,Lign.193,fig.6. The specimen,fig.2, although devoid of denticles, probably belongs to the same genus, for reasons already explained. Several species abound in the Chalk; and they are associated with teeth, which are relatively wider and shorter, and have large compressed denticles; the latter are arranged in a separate genus, namedOtodus(eared-tooth), by M. Agassiz. The specimen figuredLign.192,fig.5, representsO. obliquus; another species,Otodus appendiculatus, is abundant in the Sussex Chalk. The large, wide, triangular, smooth teeth, with trenchant edges, and destitute of lateral denticles, so common in the Chalk, are related to Lamna, and are comprised in the genusOxyrhina(Poiss. Foss.tom. v. tab. xxxiii.).

Notidanus microdon.Lign.193,fig.3.—These teeth differ remarkably from those of the other genera of Sharks. The crown of each tooth is composed of a series of sharp angular enamelled points, the first of which is the largest, and is notched on its anterior edge; the base or root is osseous, flat, with a slight longitudinal depression below the border of enamel. These teeth are comparatively rare in the Chalk. One species has been found in the Oxford Clay; and several in the Tertiary strata. Specimens occur in Hordwell Cliff.

Corax pristodontus.Lign.193,fig.1.—The teeth of the fossilCoraxchiefly differ from those of the recent genusGaleus, to which theTope, or Grey Shark, belongs, in being solid; they are of a triangular form, with a deep concavity or notch on the posterior margin, the base of which is prolonged and forms three or four angular points: the anterior edge of the tooth is finely serrated. The root of the tooth, as inNotidanus, is a broad osseous plate. There is much diversity of form in the Chalk specimens, which are all of a small size, as inLign.193,fig.1. In Sussex they are more common in the Chalk-marl than in the Chalk.

The only fossil teeth of the Shark family resembling those of the tertiary Carchariodonts, that have been discovered in the strata below the Chalk, are from the carboniferous deposits of Yorkshire and Armagh. These teeth are compressed, triangular, crenated on the edges, with large plaits or folds on the enamelled surface, towards the base of the crown. M. Agassiz refers them to a new genus, viz.Carcharopsis, with the specific name ofPrototypus.

Fossil Vertebræ of Sharks.—As the cartilaginous nature of the skeleton in this family renders it unfavourable to preservation in the mineral kingdom, the durable parts already described, and those which are ossified, are almost the only relics found in a fossil state. The dermal integument is, however, sometimes preserved; and I had a beautiful example of shagreen, composed of irregular minute hexagonal scales, one of which is represented highly magnified,Lign.185,fig.1.

In theGaleusandCarchariasthe vertebræ are more ossified than in many other genera of cartilaginous fishes, and fossil vertebræ of these sharks often occur in the cretaceous and other strata. Groups of vertebræ of a large size occasionally occur in the Sussex Chalk; they are circular, biconcave, and very short; one specimen is four inches in diameter, and one inch long; their concavities are consequently shallow. These vertebra: are composed of two shallow conical disks, which are united by their summits, at the axis, and are connected and supported by numerous wedge-shaped plates, that radiate from the centre to the periphery (seeFoss. South D.pl. xxxiii.fig.10). My collection contained a connected series of forty small vertebræ from the Chalk near Lewes, which probably belonged to the same species of Shark as the dorsal spine namedSpinax major(Poiss. Foss.tom. iii. pl. xlafig.6).

Squaloraia.In the Lias of Lyme Regis, that inexhaustible storehouse of fossil treasures, a considerable portion of the skeleton of a very remarkable fish, partaking of the characters of the Sharks and Rays, was discovered by Miss Mary Aiming, and is now in the Museum of the Bristol Institution.[527]In this fish the jaws are prolonged into a beak, like the Pristis (Saw-fish). It has the head of a Shark, with a long beak; vertebræ of the Rays; with pectoral andventral fins, almost equally developed; a tail armed with, a spine; and spinous bosses, as in the true Rays.

[527]It is figured and described by Dr. Riley, Geol. Trans. 2d ser. vol. v. pl. iv. See also Poiss. Foss. tom. iii. pl. xlii.

Fossil Pristis, or Saw-fish.—This well-known predatory fish, which is allied to the Rays and Sharks, and referred by M. Agassiz to the family ofRaiidæ, has projecting from its snout an osseous, flat, horizontal plate, or beak, equal in length to one-third of the fish, and armed on each side by a row of elongated, compressed, pointed teeth, implanted in sockets; the front margin of these teeth is convex, the posterior concave; this defence is termed the saw, and constitutes a most powerful weapon. ThePristishas also numerous small obtuse teeth on the jaws. The remains of the beak, or saw, of an extinct species of Pristis have been discovered in the Bagshot Sand at Goldsworth Hill, Surrey,[528]and three other species have been found in the London clay of the Isle of Sheppey, and the Eocene beds at Bracklesham and Hordwell.[529]

[528]Proc. Geol. Soc. vol. ii. p. 687.[529]Two teeth are figured in Dixon’s Fossils of Sussex, pl. xii.; the specimens are in the British Museum; seePetrifactions, p. 414.

[528]Proc. Geol. Soc. vol. ii. p. 687.

[529]Two teeth are figured in Dixon’s Fossils of Sussex, pl. xii.; the specimens are in the British Museum; seePetrifactions, p. 414.

Fossil Rays.—The teeth of these fishes are characterised by the extraordinary transversal development of the median teeth in both jaws. Instead of pointed teeth, they have wide, flat, tesselated dentary plates in each jaw, composed of distinct pieces, juxtaposed and connected by their margins, and united by fine sutures. In some species the teeth are equal, in others of various sizes; they present numerous modifications of arrangement, and are always disposed in symmetrical rows. In the genusMyliobatis(Eagle-ray) the teeth of the median row are of an extraordinary width, while their length does not exceed that of the lateral plates, or chevrons, which are of an irregular hexagonal form, and disposed in two or three rows on each side. There are five living species ofMyliobatis, and eighteen fossil species have been found in the Tertiary strata at the Isle of Sheppey,Hordwell Cliff, and Bracklesham Bay. I have figured a specimen of part of the upper jaw of a species (M. micropleurus,Lign.194,fig.2), in which the median teeth are very wide, and have two lateral rows of small, irregularly hexagonal, plates. The surface of the teeth in this species is smooth; but in others it is striated longitudinally (Bd.pl. xxvid.fig.14). In an allied genus,Ætobatis, from the Tertiary beds at Bracklesham, the lower jaw projects beyond the upper, and in each there is a row of flat, transverse teeth,withoutlateral plates.

Lign. 194. Fossil Teeth of FishesFig.1.—Ceratodus emarginatus:1/2nat.Lias Bone-bed.Aust Cliff.A single dental plate.2.—Myliobatis micropleurus:1/3nat.Tert.Isle of Sheppey.A series of six median teeth, with the corresponding lateral teeth.3.—Pycnodus Mantelli.Weald.Tilgate Forest.This specimen consists of thevomerinebone, with a median row of flat, arched teeth, and two rows on each side of elliptical teeth, arranged alternately.

To this notice of the fossil Rays, we may add, that a giganticTorpedohas been discovered in that celebratedlocality of Ichthyolites, Monte Bolca: and that Sir Philip Egerton has recently enriched his matchless collection of fossil fishes, by a perfect Ray, from Mount Lebanon. It is figured and described in theQuarterly Geol. Journ.vol. i. pl. v. p. 225, under the name ofCyclobatis oligodactylus. It is a small species, resembling the common Rays in its general appearance, but is surrounded by a broad, flexible, cartilagino-membranous fin; the skin is smooth, the teeth and eyes are small, the tail is slender; there are no traces of dermal spines, tubercles, or defensive weapons. In many points of structure it resembles the Torpedo; and may possibly, like that fish, have possessed an electric organ. It is 31/2inches long, and 3 inches wide across the expanded fins.

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

Thefishes we have hitherto examined belong to the first order, thePlacoidians; we now pass to the fossil remains of the second order, theGanoidians, 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 theGanoids, properly so called, and of theSauroids, or lizard-like fishes.[530]

[530]The fishes of these orders are described inPoiss. Foss.tom. ii.

The first family, termedLepidoides, 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 (seep. 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 thePermian and Carboniferous strata; namely,AmblypterusandPalæoniscus.[531]

[531]For the characters, affinities, and distribution of these and the allied genera of the Heterocerque Ganoid fishes, see Sir P. Egerton’s Memoir in the sixth volume of the Quarterly Geological Journal.

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 vertebræperfect. 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.

[532]Professor Owen states that a similar condition of the spinal column obtains in the fossil Microdonts.—Rep. Brit. Assoc.1846.

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.

[533]See Professor Sedgwick on the Magnesian Limestone. Geol. Trans. 2d ser. vol. iii.; and Prof. King’s Monograph on the Permian Fossils, published by the Palæontographical Society.[534]Dr. Hibbert’s Memoir on the Fossils of Burdie-house.[535]Geology of Massachusetts, by Professor E. Hitchcock.

[533]See Professor Sedgwick on the Magnesian Limestone. Geol. Trans. 2d ser. vol. iii.; and Prof. King’s Monograph on the Permian Fossils, published by the Palæontographical Society.

[534]Dr. Hibbert’s Memoir on the Fossils of Burdie-house.

[535]Geology of Massachusetts, by Professor E. Hitchcock.

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.

Lign. 195. Outline of the Dapedius.Lias.(1/2nat. size).

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 inLign.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 laterallyby short processes; as in many other ganoid fishes. It belongs to the Lepidoids.

TheTetragonolepisis a Liassic fish, very similar in shape to the Dapedius. It was formerly grouped with theLepidoidei, 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-menfishes' scalesandeyes. The collectors of the last century used to term the obtuse circular teeth of this and the related generaBufonites, from a supposition that they were formed in the heads of toads. These relics belong to an extinct genus namedLepidotus, 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 (seeLign.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 fluviatiledeposits, as in the Purbeck and Wealden strata; and it is probable they inhabited the rivers or sea-coasts, and not deep waters.

[536]Poiss. Foss.tom. ii. p. 233.

Lign. 196. Scales and Fin of Lepidotus Mantelli.Wealden.Tilgate Forest.(Nat. size.)Fig.1.—Scale, with a single process of attachment.2.—One of the scales of the dorsal line.3.—Scale (external surface), with a bifurcating process of attachment; the enamelled portion (which alone is visible when the scales are in their natural position on the animal) has longitudinal grooves or folds.4.—Scale (viewed on the inner surface), having a bifurcating process of attachment and a tooth or projection on each side, to connect the scale laterally with the adjoining scales.5.—The front ray of the dorsal fin, covered with two rows of enamelled scales, and another ray behind it.

The scales and teeth figuredLign.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 figuredFoss. Til. For.pl. v. and x.; considerableportions 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, andPl. 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.

Lign. 197. Portion of the Jaw of Lepidotus.Wealden.Tilgate Forest.(Nat. size.)This specimen shows three successional teeth beneath a row of teeth in use.

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: seePl. 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 inPl. 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]

[537]Odontography, p. 70. See the beautiful representation of this structure, pl. xxxi.

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 representedLign.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, figuredPetrif.Lign.38, is sprinkled with the scales and minute teeth of this fish.

[538]Fine specimens of this fish are in the British Museum (Petrifactions, p. 429), and in the Museum at Dorchester.

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 genusPycnodus(P. rhombus), from the Jura limestone, at Torre d’Orlando, near Castellamare, is figured in thefrontispieceof the first volume of this work; and a vomerine bone, with teeth, from Tilgate Forest, inLign.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.

[539]The intermaxillary, palatine, and vomerine bones compose the vault or roof of the mouth; the vomer occupying the middle; the intermaxillary the front; and the palatine bones the sides; the premandibular bones belong to the lower border of the mouth.

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]

[540]An extraordinarily perfect lower jaw of a Gyrodus is to be seen in the British Museum (Foss. Brit. Mus.p. 439).

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 nearRjeff, 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 acoarse 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.

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