Chapter 33

[509]For illustrations of the teeth of fishes, seeplate iv. figs. 1, 2, 8, 10, andLigns.189,191-194,197,198,202,205;Foss. Brit. Mus.p. 449; andLy.figs. 236, 308, 324, and 383.

The mode of arrangement and attachment of the teeth, is as diversified as their forms. In some species all the teeth are of one type, and disposed in somewhat of a serial order on both sides of the jaws; but in a large proportion of fishes there are several kinds of teeth, which are implanted not only in the jaws, properly so called, but on the bones which form the cavity of the mouth, the arches of the palate, tongue, &c.; and it is peculiar to this class of vertebrata to present examples of teeth developed in the median line (along the middle) of the mouth, as in certain species of Rays; or crossing the symphysis (thefront line of unionof the two sides) of the lower jaw, as inMyliobates[510](seeLign.194,fig.2). In some species the teeth are implanted in sockets, to which they are attached only by the soft parts, as in the rostral teeth of theSaw-fish; some have hollow bases, supported upon bony prominences, which rise from the base of the socket; as in several fossil teeth from the Chalk. "But by far the most common mode of attachment is by a continuous ossification between the dental pulp and the jaw,"[511]the teeth being thus anchylosed to the bone. In the Sharks the osseous bases of the teeth are attached by a ligamentous substance to the tough, dense crust, which covers the cartilaginous jaws; the teeth of these fishes are therefore generally found detached in a fossil state, in consequence of the decomposition of this substance.

[510]Odontography, p. 5.[511]Op. cit. p. 6.

[510]Odontography, p. 5.

[511]Op. cit. p. 6.

The teeth are composed of a dense, osseous material, of a finely tubular structure, termeddentine; which, in many species, forms on the external surface of the tooth a layer offirmer texture, with a glossy surface, resembling enamel. The essential character of their organization is to have a pulp or medullary cavity, or cavities, filled with a plexus of blood-vessels and nerves, from which the minute tubes composing the dentine radiate.[512]The differences observable in the size, mode of ramification, and distribution of the medullary cavities or canals, and thecalcigerous tubes,[513]as revealed by microscopic exploration, constitute important distinctive characters; particularly in the examination of the fossil teeth of extinct fishes. In some teeth the dentine is traversed by equidistant, parallel, medullary canals; in others, these channels frequently subdivide, and their branches anastomose with each other. In some the medullary canals form a reticulated, or net-like structure in the dentine, the meshes of which are occupied by calcigerous tubes, and cells; often producing a dendritical appearance, as in the tooth of a fossil fish namedDendrodus. "In the highest type of structure, the dentine consists of a simple medullary cavity or canal, and a single system of calcigerous tubes, which radiate from the central or sub-central pulp-cavity, at right angles to the periphery of the tooth" (Owen), as in the teeth of the extinct Sauroid (lizard-like) fishes. A continued succession of teeth takes place during the life of the fish, and we often find in fossil specimens a series of successional teeth beneath the row in use; as in the fragment of a jaw ofLepidotus, from Tilgate ForestLign.107.

[512]See Owen’s Odontography; and Tome’s Dental Physiology.[513]Calagerous tubes; so named because they are composed ofcalx, or lime.

[512]See Owen’s Odontography; and Tome’s Dental Physiology.

[513]Calagerous tubes; so named because they are composed ofcalx, or lime.

Skeletons of Fishes.—The skeletons of the animals of this class differ so remarkably in their relative degree of firmness and elasticity, in consequence of peculiar modifications of their constituent substance, as to form two grand divisions; one of which is termed theosseous, the other thecartilaginous. The essential difference in the skeletons of these two groups consists in the presence or absence of earthy matter (phosphate and carbonate of lime) in the materials of which they are constructed. In the cartilaginous fishes, the skeleton is cartilaginous and transparent; but in some species, the skin has dense osseous particles or plates on the skin, as in theRays; and in others, the head and body are protected by large osseous scutcheons, as in theSturgeon. There is also an intermediate group of fishes, termed thefibro-cartilaginous, in which the skeleton contains lime, but in a much less proportion than in the true osseous fishes. In some genera, certain portions of the skeleton, as the bodies of the vertebra, are cartilaginous, while the spinous processes, ribs, &c. are osseous; these characters are of considerable importance in the investigation of the fossil remains of fishes, as we shall hereafter have occasion to demonstrate.

The skeleton consists of thecraniumor skull, which is composed of numerous bones,—the jaws, and bones of the tongue,—the osseous frame-work of the organs of respiration, consisting of the bones, rays, and arches that support the gills, and theopercula, or covers which close over the branchial apertures,—and of the vertebral column, formed of numerous dorsal and caudal vertebræ, with the ribs and other appendages; there are no proper cervical vertebræ, or spinal bones of the neck.

The branchial arches are in general four or five on each side, and are attached above to the cranium, and below to a chain of small bones, by which they are connected with theos hyoides, or bone of the tongue. The opercular bones, composing the cover or lid of the opening of the gills, consist of three pieces on each side, and are distinguished by the names,opercular,pre-opercular, andsub-opercular, according to the situations which they respectively occupy.

Thevertebræare double hollow cones,[514]not unlike an hour-glass in form: the interval between two of these bones is filled up, in the living state, by a gelatinous fluid. Along the upper part of each vertebra, there is an annular cavity, which in the united vertebral column forms a canal for the spinal marrow; the posterior dorsal and caudal vertebæ have also a channel below, for the passage of the large blood-vessels.

[514]There are certain exceptions; thus in theLepidosteusthe vertebral column is a series of ball-and-socket joints, the convexity being anterior, as in the land Salamander, and in the fossil reptile known as the Streptospondylus.

There are likewise bones analogous to some of those which enter into the composition of the extremities, chest or thorax, and pelvis of the higher vertebrata; but which it is not necessary for our present purpose here to describe.

Of the organs of vision some fossil remains also occur. The sclerotic coat, or capsule of the eye, being bony in fishes, is often preserved; and in several chalk specimens I have found it occupying the orbit.

In addition to those durable parts of fishes, already mentioned, as likely to be met with in a fossil state, the bones calledotolithes(ear-stones) must be enumerated. These calcareous bodies are found in the membranous labyrinth of the organs of hearing; and, although more or less developed in the ear-bulb of all animals, they are larger and of more definite forms in the higher osseous and cartilaginous fishes. Theotolithesare supposed to assist in communicating more vivid impressions of sounds to the extremities of the auditory nerves; they are stony in most aquatic animals, and friable or pulverulent in those that live on land. Smooth, oblong otolithes are not uncommon in the Crag deposits of Norfolk and Suffolk; and minute ear-bones are found in the Barton Clay.

Tails of Fishes.—The tail, as we have previously mentioned,is the chief instrument of progressive motion in these animals; it assumes two principal modifications. In the greater number of the existing species the vertebral column terminates in a triangular plate of bone (formed by the fusion of the last few vertebræ), to which the caudal fin is attached symmetrically; and its figure is either rounded, or divided into two equal lobes or branches; these tails are termedhomocercal, i. e. even-tail. In the second modification the vertebral column towards its extremity diverges from a straight line, rises up, and is prolonged into the upper lobe of the tail; the caudal fin appearing like a rudder, and its low’er lobe, being destitute of vertebræ, is proportionably very feeble and small, as in the Shark and Dog-fish: this form of tail is calledheterocercal, i. e. unequal-tail (seeFoss. Brit. Mus.p. 421; andLy.figs. 340, 341). In the embryonic state the tail in all fishes is heterocercal, and it becomes homocercal in the progress of development in those genera which have this type of the caudal appendage. But few of the existing species have the heterocercal tail, while it is found in all the fossil fishes that occur in the ancient secondary strata; namely, the Magnesian limestone, and antecedent deposits. The rounded and equal-bilobed, or homocercal, tails, are seen in the fishes from the Chalk,Wond.pp. 347, &c.; and in the Wealden Lepidotus,Lign.186; and the unequal or heterocercal tail is shown in the Amblypterus from the Carboniferous strata,Lign.187.

Lign. 186. Lepidotus.Wealden.(1/6nat. size.)(Showing the Homocercal Tail.)

Lign. 187. Amblypterus.Carboniferous.(1/4nat. size.)a.The heterocercal tail.

In the Annals of Nat. Hist, for 1848, p. 304, Prof. M’Coy has described and figured an intermediate form of tail, which he regards as characteristic of theDiplopterus(of the Old Red Sandstone) and its allies: this the Professor terms theDiphycercaltail.

From this brief summary of the essential characters of those durable parts of the organization of fishes, which most frequently occur in a fossil state, we pass to the investigation of some illustrative examples of this class of organic remains. But before describing any entire specimens, it will be expedient to notice the separate fins, and teeth, which abound in many deposits; in some instances occurring in connexion with other parts of the skeleton, but more generally detached, and constituting the only evidence of the existence of numerous extinct species and genera. The greater part belong to the first order—the Placoidians (Poiss. Foss.tom. iii.), and to the families ofSharksandRays. The osseous dorsal rays of cartilaginous fishes (namedIchthyodorulites(fossil-fish-weapons) by Dr. Buckland and Sir H. De la Beche) first demand our notice.

Ichthyodorulites.Lign.188.—This name is appliedto the fossil spines, or rays, of dorsal fins, of which numerous species occur in the Secondary deposits; they belong, for the most part, to extinct cartilaginous fishes of theCestracionidæandHybodontidægroups. In the osseous tribes the dorsal spines have at their base two articular processes, by which they are united to the osselets that support them, as in theSilurus; but in the cartilaginous, they have no articulations at the base, and terminate in an obtuse point, which is implanted in the flesh; the posterior margin is grooved almost to the upper extremity. They are of a fibrous, osseous texture. The common Spinax, or Dog-fish (Acanthias vulgaris), has a spine of this kind in the front of each dorsal fin. The rays of the Sharks are compressed, and some have rows of teeth on the posterior margin; in the genusCestracion(Port-Jackson Shark), these organs are strong, triangular, straight, pointed, rounded in front, flat at the posterior face, and widest at the base; in the Hays they are flattened or depressed.

These spines are generally capable of being elevated and depressed, and not only serve the purpose of defence, but, in many instances, afford support and protection to the soft rays of the fin; forming, as it were, a moveable mast, by which the sail can be spread out or lowered at pleasure.

In illustration of this subject, I would first direct attention to the beautiful fossil, figuredLign.188,fig.1, which was discovered in the Chalk near Lewes, and is figured, of the natural size,Foss. South D.tab. xxxix. This ray, or spine, belongs to one of the Cestraciont fishes (Ptychodus), whose teeth are so abundant in the Chalk, and will presently be described. It is composed of fourteen thick, flat, osseous rods, or strands, intimately united together, with longitudinal furrows or sutures on the surface. The anterior margin is embossed, and the projections form on the sides wide, rounded ribs, and transverse depressions. Towards the base of the posterior part, there are large osseousfibres inserted vertically and obliquely, which appear to have been processes of attachment. The rods, or plates, are parallel With the posterior margin, and each terminates in a rounded extremity, or boss, on the front edge of the spine. This ray is wider at its base than at the superior part the anterior margin is oblique, and the posterior straight. The surface, where entire, is covered with a dense osseous substance, which is finely engrailed.[515]

[515]This specimen is figured inPoiss. Foss.; but it is represented too short, from the two portions being drawn as if they were connected, without any interval between them, as inLign.188. It is in the British Museum. SeePetrifactions, p. 450.

Lign. 188. Dorsal Rays of Sharks.Sussex.Fig.1.—Ptychodus spectabilis.1/5nat.Chalk.Lewes.1a.—Portion of aDorsal Ray, with oblique serrated sutures.Chalk.Lewes.2.—Ptychodus Gibberulus.1/5nat.Chalk.Lewes.3.—Hybodus subcarinatus.nat.Weald.Tilgate Forest.

In 1850 I discovered in the Plastic Clay of Castle Hill Newhaven, a dorsal fin of Ptychodus, with eight vertebræ. A nearly entire fin-ray of this species, three feet in length,has recently been discovered by Mr. Charles Potter, of Lewes, in the Chalk near that town. The remains of another ray, of equal proportions, were found near it; and these dorsal spines might have belonged to the same individual, for there are no reasons to forbid the supposition that the Ptychodus had two dorsal fin-rays. The length of these spines necessarily indicates a very large fish.

A smaller species of Ichthyodorulite, also found in the Lewes Chalk, is distinguished fromP. spectabilisby its osseous plates contracting towards their extremities, and terminating more suddenly on the front margin, producing gibbosities less acute and more distant than inP. spectabilis; this species is namedP. gibberulus: seeLign.188,fig.2.[516]

[516]This fossil is figured of the natural size, Foss. South D. pl. xl.fig.3.

The bony plates of these fins are occasionally found lying in irregular groups in the Chalk, as if the fin had partially decomposed and the plates separated. In one example, the rays are split asunder by a piece of bone, apparently a portion of a long pointed tooth, firmly impacted between them; as if the fish had been seized by some enemy, and had escaped, with the tooth of its adversary in its fin. Very fine specimens have been found at Charing, Kent, by W. Harris, Esq. F.G.S.

In the fragment of an Ichthyodorulite from the Lewes Chalk, a remarkable structure is displayed; the osseous plates are united laterally by smooth, longitudinal lines, as in those above described; but they are also traversed by numerous oblique, finely-serrated sutures.Lign.188,fig.1a.

The Chalk contains rays of other species ofPtychodus, as well as of some allied genera. Of these, the most remarkable are smooth, arched, pointed spines, having a shallow posterior groove, with an enamelled surface, marked with fine longitudinal striæ, and frequent, parallel, oblique lines. These, according to Sir P. Egerton, belong to a trueCestracion(seep. 584): they were first figured and described by me (Foss. South D.tab. xxxiii.fig.5) as belonging to theAcanthias major, and were subsequently assigned to the genusSpinaxby Prof. Agassiz (Poiss. Foss.iii. p. 62).

It may be necessary to remark, that the fins first described have been referred to the fishes which yielded the large grooved teeth so common in the Chalk (seePl. VI fig. 2) in consequence of their affinity to existing species, which have similar fins and teeth; and from the circumstance that the Sharks of the genusLamna, whose teeth also abound in the Chalk, have no dorsal rays of this kind; still the proof of identity remains to be discovered. In one specimen only have I observed indications of any other part of the skeleton; it is a spine ofAcanthias major, the base of which rests on several dorsal vertebræ (Foss. South D.tab. xxxiii.).

Hybodus subcarinatus.Lign.188,fig.3.—The fishes of another extinct genus of Sharks, termedHybodus, from the gibbous form of the teeth, were also provided with dorsal spines, which may be readily distinguished from the preceding. These Ichthyodorulites are generally long, slightly arched, and terminate in a point at the extremity; the base, which was implanted in the flesh, is deeply grooved, and much prolonged, being sometimes equal to one-third of the entire length. The surface is marked with strong longitudinal ridges, parallel with the anterior margin which is rounded and laterally compressed. The posterior edge, which is more or less flat, has, towards the base two rows of sharp arched teeth, which gradually approach ’each other, and blend into one line on the upper part of the ray There are numerous species of this genus in the Oolite and Lias. I have found one species in the Chalk and a few in the Wealden. The small Ray figuredLign.188fig.3, is from Tilgate Forest, and displays the usual characters of these fossils. From specimens discovered in theLias, associated with the teeth, it appears that theHybodushad two dorsal fins, each furnished with rays, as in the recent Dog-fish.

The microscopic structure of these rays is stated by M. Agassiz to resemble that of the teeth: in some there is a pulp cavity, which occupies the centre of the spine, and is surrounded by dentine, in which the calcigerous tubes radiate direct to the surface; the external enamel is a layer of dentine, in which the medullary canals are wanting.

In the strata below the Lias there are numerous Ichthyodorulites, some of a large size, belonging chiefly to the Cestracion family, and of extinct species, not observed in more recent deposits. Thus there are several species of dorsal rays (namedOnchus, from their hooked form,) that are wide at the base, and bent backwards, with the posterior margin destitute of teeth, in the Carboniferous, Devonian, and Silurian formations; also immense compressed spines, having small teeth on the posterior margin, and the surface covered with longitudinal striæ, and finely toothed, transversely; hence termedCtenacanthus, or pectinated-spine (Murch. Sil. Syst.p. 596).

The fossil spine, namedOrthacanthus(Poiss. Foss.vol. iii. pl. xlv.), and found in the Coal of Manchester, has been discovered in connexion with the body of the fish to which it belonged in the Carboniferous deposits of Ruppersdorf in Bohemia (Geol. Journal, vol. v. part ii. p. 23).

Some Ichthyodorulites have the surface richly ornamented with stellular tubercles, and are termedAsteracanthus, or starry-spine; there are very large fin-rays of this kind in the Wealden, Purbeck, Oolites, and Lias.[517]

[517]For particular information on Ichthyodorulites, consultPoiss. Foss.tom. iii. chap. i. About seventy species are enumerated.

The Ichthyodorulites of theRayshave no cavity like those of theSharks, and are of a depressed form, and more or less flattened; they are armed with teeth along theirexterior margins, and not on the posterior edge, as in the latter family.

Fossil Teeth of Fishes.—From the durable nature and striking appearance of many of the fossil teeth of fishes, and their prodigious numbers in some deposits, they are familiar objects to the collector. By far the largest proportion of the detached teeth belongs to various species and genera of that most numerous, and widely distributed family of voracious fishes, theSharks. In the Tertiary strata teeth of this kind occur of a very large size; in the Chalk many species abound, particularly of the lanceolate and compressed forms, and of the rugous, mammillated, palatal teeth, commonly termedpalates. As we pass to the more ancient formations, teeth of different forms prevail; and those which approach the recent types are either very rare or altogether absent. We will select some examples of the different genera in illustration of this subject; the previous observations on the form and structure of the recent teeth render but few introductory remarks necessary.

Fossil Teeth of Sharks.—The fishes of the Shark and Ray families belong to thePlacoidorder; the scales in the former consist of enamelled plates and tubercles, forming ashagreensurface; and in the dermal integument of the latter they appear as spines and bosses, irregularly disposed.

Notwithstanding the diversity in appearance of the teeth of Sharks, they all possess one essential character of structure, namely, a base, or osseous root of variable form, which is implanted in the integuments; and a crown, or external portion, which projects into the mouth, is covered with enamel or compact dentine, and assumes numerous modifications, by which the fossil genera are characterized. These teeth are never imbedded in sockets, nor united to the dentary margins of the jaws; they only adhere to the integuments of the mouth, and the covering of the maxillæ; they possess, in most of the Sharks, great mobility. Theyare generally disposed in rows; the anterior ones, being first used, fall out, and are replaced by those on the inner series. New teeth are also continually formed behind those in use, and advance successively towards the anterior rows as the latter are shed, and in their turn occupy the front rank. (See Cyclop. Anat.Art.Teeth.) An examination of the fossil and recent teeth of Sharks and Rays proves that the prevailing existing generic types have but few, if any, representatives in the fossils, except in those which belong to the Tertiary and Cretaceous formations; while the genera that appear isolated, as it were, in the present seas have numerous analogues in the Secondary strata.

The fossil teeth of this family may be divided into two grand divisions; namely, those which are more or less of a polygonal, obtusely conical, or depressed form, having a tesselated arrangement in the mouth; and those of a triangular, lanceolate shape, with cutting, or serrated edges, disposed in a series of rows on the jaws. The teeth of the first group (Cestracionidæ) have most analogy to those of the living genusCestracion(Port-Jackson Shark); the second (Sgualidæ) to theSharks, commonly so called.

The Cestracion is the only living representative of a family of squaloid fishes of a peculiar type, whose remains occur in almost the earliest fossiliferous deposits; it inhabits the seas of New Holland and the southern coasts of China. The jaws of the Cestracion are relatively very large, and are armed with numerous rows of teeth, essentially of two kinds; those situated anteriorly, or towards the front of the mouth, being adapted for seizing and retaining the food, and the posterior ones for crushing and bruising. The prehensile teeth are sharp, angular, and pointed: the others are obtuse, polygonal, enamelled, and disposed in oblique rows along the margins and inner surface of both jaws; there are sometimes sixty in each jaw (seeBd.ii. pl. xxvii 11.fig.A). Fossil teeth of this type are exceedingly numerous in the Chalk, Lias, &c. but are very seldom found in juxtaposition;the decomposition of the cartilaginous integuments in which they are imbedded, having, in most examples, occasioned their displacement and dispersion; specimens, however, are occasionally discovered, in which numerous teeth, of various sizes, are disposed in mosaic, in their natural relative positions.

The extinct forms of this family (Cestracionidæ) are known almost only by their teeth; and according to the shape, structure, and sculpture of these organs, M. Agassiz has arranged them into several genera. They occur in most of the fossiliferous deposits.

Cestracion canaliculatus.—The teeth of a fish belonging to the existing genus have been discovered in the Chalk of Kent; they are figured and described by Sir P. Egerton in the beautiful work by Mr. Dixon.[518]This unique specimen consists of a group of thirteen posterior molar and three or four detached prehensile anterior teeth, imbedded in a block of chalk about two inches square. The chief distinction from the teeth of the recent Cestracion is in the presence of a large medullary canal which traverses the base of each tooth: hence the specific name.

[518]Dixon’s Fossils of Sussex, &c. p. 365, tab. xxxii.fig.8. From the t examination of a specimen lately found at Lewes, Sir P. Egerton has been led to assign to this species the spine formerly described asSpinax major.

Acrodus(ridge-tooth)nobilis.Lign.189,fig.4,Ly.p. 275,fig.307.—In the Lias and Oolite, oblong enamelled teeth, having the surface of the crown covered with fine radiating grooves and striæ, are well known to collectors, in many parts of England, by the name offossil leeches, from a fancied resemblance to a contracted leech. They belong to an extinct genus of Cestracionts, namedAcrodusby M. Agassiz. The crown of the tooth is enamelled, and covered with transverse grooves, which diverge from a longitudinal furrow; the base is in the form of a parallelogram inclined on its inner side. These teeth were inserted along thejaws in oblique series, their longitudinal direction corresponding with that of the bones which supported them; in their natural position, the extremity of a hinder tooth was enclosed between the two next anterior teeth. A beautiful group is figuredBd.ii. pl. xxviie.[519]

[519]The microscopical structure of the teeth ofAcrodusis well shown in the "Odontography," pl. xiv. xv., and beautifully illustrates the relation of dentine to bone.

Lign. 189. Fossil Teeth of Sharks.Fig.1.—Ptychodus Mortoni.(G. A. M.)Cret.New Jersey.2.—Psammodus cinctus.(Ag.)Mt. L.Bristol.3.—Orodus cinctus.(Ag.)Mt. L.Bristol.4.—Acrodus nobilis.Lias.Lyme Regis.

Ptychodus(wrinkle-tooth).Pl. VI. fig. 2;Lign.189, andLign.191.—The palatal teeth, which occur more or less abundantly in almost every chalk-pit, and are known by the name of "palates," belong to several species of the genus Ptychodus. A very common form is figuredPl. VI. fig. 2; and microscopic views of vertical and transverse sections, as seen by transmitted light, are shown in figs. 2b, 2c. Groups of these teeth, somewhat naturally arranged, and varying in size and form according to the situations they occupied in the jaws, are occasionally found: one specimen in theBritish Museum, and formerly in my collection, contains more than 120 teeth. In general they occur in a very perfect state, with the osseous base and enamelled crown entire. The dorsal rays or spines previously described (p. 577), are sometimes found with the teeth, and belong to fishes of the same genus.

These teeth are of an angular form, and more or less square, the crown is wider than the root, which is obtuse, truncated, and depressed in the centre; the enamelled part of the tooth is expanded at the edges, and forms in the centre a flattened or slightly convex mammillary projection, which is traversed by large, acute, transverse, parallel ridges. The borders are granulated, and the sides of the projection marked with deep vertical plicæ or folds; this description particularly applies to the species namedP. polygurus, figured inPlate VI.Dr. Buckland has represented a fine group of these teeth,Bd.ii. pl. xxvi′. Another common species (P. decurrens) is distinguished from the former by the connexion between the large furrows on the crown and the granulations on the expanded border, which diverge from the outer edge of the large folds to the margin of the enamel.

The microscopic structure of these teeth presents the same congeries of medullary and calcigerous tubes as those of the recent Cestracion: seePlate VI. figs. 2b, 2c.

The teeth of a species of Ptychodus occur in the arenaceous strata of the Chalk-formation in New Jersey, which possess the essential characters of the European types, but differ from them in their configuration; the only specimen I have seen is figuredLign.189,fig.1; it was presented to me by Dr. Morton. The enamelled crown forms a conical projection, traversed by large inosculating ridges, which radiate from the summit towards the margin.[520]

[520]I have named itP. Mortoni, in honour of my distinguished friend, the eminent American naturalist and physician, Dr. George Morton, of Philadelphia, by whom it was discovered.

Psammodus[521](sandy-tooth).Pl. VI. fig. 1;Lign.128,fig.2.—To this genus are referred the fossil teeth of the extinct Cestracionts, which have the crown formed of small vertical tubes, with the grinding surface more or less smooth, and presenting only a punctated or sandy appearance.[522]These teeth are generally flat or slightly convex, and of a square or oblong form; the base is osseous, and as large as the crown. Two species are figured,Lign.189,fig.2, andPl. VI. fig. 1a. A magnified vertical section of the crown, displaying the medullary canals and radiating calcigerous tubes, is representedPl. VI. fig. 1b, and a transverse section,fig. 1c; they are thin slices of a tooth,P. porosus, from the Black Rock (Mountain Limestone), near Clifton, viewed by transmitted light. The large, flat, quadrilateral, oblong teeth that abound in the Stonesfield Slate belong to theStrophodus magnus.

[521]See Odontography, pl. xviii. xix.[522]Ibid. vol. i. p. 59.

[521]See Odontography, pl. xviii. xix.

[522]Ibid. vol. i. p. 59.

There are several kinds of fossil teeth which possess the same essential structure as those of Psammodus, but differ in their external characters; these are referred to other genera by M. Agassiz. ThusOrodus,Lign.189,fig.3, comprises those elongated teeth in which the centre of the crown forms an obtuse transverse cone, traversed by a ridge from which oblique furrows diverge transversely towards the circumference. Similar teeth, but with a smooth, obtusely conical crown, are referred to the genusHelodus. Those with the crown compressed and elevated, with a sharp edge, and with the base surrounded by concentric folds, constitute the type ofChomatodes. A similar crown, but subdivided by deep transverse ridges into dentations, characterises the genusCtenoptychius.

Ceratodus(horn-tooth)emarginatus.Lign.194,fig.1.—Very curious dental organs, possessing a structure analogous to that of the teeth of Psammodus, are found in theBone-bed of the Lias; they consist of consolidated plates instead of separate teeth; there was probably but one plate on each side the jaws. The upper margin is generally undulated, and more or less worn by use. These dental plates are composed of two distinct layers; the lowermost portion, or root, is an osseous, reticulated tissue, as in cartilaginous fishes in general; and the upper consists of dentine, with minute parallel vertical tubes, as in Psammodus; these tubes are a continuation of the medullary tissue of the osseous root.

One species occurs in the Great Oolite at Stonesfield, and very many forms abound in the Bone-bed at Aust Cliff, near Westbury on Avon: and in the Trias (bone-bed) of Germany the teeth of several species of Ceratodus are very abundant.

The fishes to which these fossil teeth, referred toCeratodus, belonged were most probably Cestracionts; the ray-spine known asNemacanthusis provisionally assigned to them.

Edaphodon.Lign.190andLign.191,Ly.p. 276,fig.309.—The Chimæroid fishes, though formerly placed with the Plagiostomes (Sharks and Bays), constitute a distinct group, of which there are but two recent genera, though several occur in a fossil state. Their dental organs are very peculiar. Their mandibles are furnished with two or more pairs of oblong teeth, composed of long hollow cylindrical columns, placed nearly at right angles to the grinding surface, which is pitted with minute depressions. These teeth are never shed, but are persistent, and grow on through life, as in the Rodentia, exhibiting in this respect a striking contrast with those of the Sharks, which are feeble and numerous, and constantly replaced by rows of successional teeth.

Fossil teeth of several species, some much larger than the recent, have been found in the Tertiary, Cretaceous, and Oolitic deposits. The first British specimen was discovered in the Chalk-marl at Hamsey, in 1820, by myself; but itsnature was not suspected until more perfect examples were obtained from the Kimmeridge Clay at Shotover by Sir P. Egerton, and were submitted to Dr. Buckland, who subsequently ascertained their characters and relations by an examination of the dental organs of a recentChimærain the Museum at Leyden in 1835.[523]

[523]Proceedings of the Geological Society of London, vol. ii. p. 209.

Lign. 190. Mandible of Edaphodon Mantelli.Chalk.Lewes.(1/2nat. size.)

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