Chapter 37

[579]ThePleurodontsare those lizards in which the teeth are anchylosed to thesideof the dentary bone;Acrodonts, those with the teeth fixed to the upper margin orridgeof the jaw-bone;Thecodonts, those having the teeth implanted, either loosely, or anchylosed to the walls of their sockets. Lacertians are also said to bePleodont(having solid teeth), orCœlodont(hollow-toothed).

In reptiles, we have, therefore, five essential modifications in the attachment of the teeth; namely, in distinct sockets; in a continuous groove or furrow; attached laterally by the shank to an alveolar parapet; anchylosed by the base to a shallow socket; and attached to an osseous support, without sockets or an alveolar plate.[580]

[580]See the beautiful exemplification of this subject, and the comparison between the transitory stages of the human teeth in their progress of development, discovered by Mr. Goodsir, with the permanency of these conditions in reptiles. Odontography, p. 182.

Lign. 205. The Lower Jaw of an Iguana.(From Barbadoes.)Fig.1.—The right branch of the lower jaw of an Iguana; viewed onits inner aspect.Nat. size.a.Dentary bone.b.Opercular bone.c.Complementary or coronoid bone.d.Surangular bone.e.Angular bone.f.Articular bone.2.—The external aspect of the same.3.—3.—Inner aspect of three teeth (magnified) attached to the alveolar parapet; with the germ of a successional tooth at the base of the middle tooth; and the sockets of other germs at the bases of the outer two fully formed teeth.4.—External view of the crowns of three teeth; slightly magnified.

The intimate structure of the teeth consists of a simple pulp-cavity, surrounded by dentine, which is permeated by extremely minute calcigerous tubes, radiating at right angles to the periphery, or external surface of the tooth. One essential modification of this structure consists in the intermingling of cylindrical processes of the pulp-cavity, in the form of medullary or vascular canals, with the finer tubular structure; as in the tooth of the Iguanodon,Pl. VI. figs. 4band4c. But another modification is that to which allusion was made when describing the teeth of the Lepidosteus (seep. 616); in this mode, the dentine preserves its normal character, but the external cement and surface of the tooth are deeply inflected in longitudinal folds around the entire circumference; and this structure is accompanied with corresponding extensions of the pulp-cavity and dentine into the interspaces of these inflected and converging folds.[581]This organization is shown, in its simplest form, in the transverse section of the base of a tooth of the Ichthyosaurus,Pl. VI. fig. 9; and attains its most complicated condition in that of the Labyrinthodon,Pl. VI. figs. 3a, 3b, 3c.

[581]There is a marked difference between the internal structure of the teeth of true Saurians and of Sauroid Fishes. In the former, as well as in theEnaliosauria, the dentine consists of tubes radiating from a slender central pulp-cavity to the periphery of the tooth, without any intermixture of vascular canals. In the sauroid fish (Dendrodus) the central pulp-cavity is produced into numerous irregular canals, from which vascular sinuses radiate to the periphery, sending off branches generally at right angles throughout their entire course; thus, there is an extensive distribution of the vascular system through the body of the tooth, which does not exist in any saurian reptile; the nearest analogy is in the labyrinthine teeth of the gigantic fossil batrachians. (Owen: Odontography; and Art.Teeth, Cyclop. Anat.)

With regard to the mode of development of the teeth, we must briefly state, that the germ of the new tooth is always produced at the side of the base of the old one; that in its progress of growth it presses against the tooth it is destined to supplant, occasions the progressive absorption of the fang, and ultimately displaces its predecessor; in some instances, by splitting the crown of the tooth; in others, by casting it off, according to the oblique or direct position the new tooth attains in its progress, in relation to its predecessor. Thus, in the teeth of the Crocodile, the new tooth is generally found immediately under the conical apex of the crown, and beneath the former a second successional tooth appears, like a series of thimbles of varioussizes placed one upon another; for in reptiles the production of new teeth is unlimited. But in thePleurodontlizards, the new tooth makes its way obliquely, and the crown is often shed entire.Lign.205,fig.3, exemplifies the situation of the successional teeth in the Iguana.

Lower Jaw of Reptiles.—It is well known that the lower jaw in mammiferous animals is composed of a single bone on each side; and that in many genera these pieces become united in front, and form but one bone in the adult state. But in reptiles, the lower jaw consists of six distinct bones on each side, as inLign.205; and these undergo various modifications of form and arrangement in the different genera. These bones are distinguished by names which have reference to their office and situation, and are as follow:—Lign.205,a, thedentarybone, supporting the teeth;b, thesplenialoropercular;c, thecoronoidorcomplementary;d, thesur-angular;e, theangular;f, thearticular, which forms the upper portion of the jaw, and includes the condyle. The form and disposition of these bones in the Iguana, and other true lizards, are shown inLign.205; but they differ materially in the Crocodile, Ichthyosaurus, and other genera. We must restrict our comments to this short notice, which, however, will suffice to enable the collector who discovers a fragment of a lower jaw, with any traces of the structure above described, to determine that it is reptilian; and if any portion of thedentarybone remains, indications may be obtained of the family, and perhaps genus, to which it belonged.[582]

[582]To obtain a correct knowledge of the osteological structure of fossil Reptiles, the student should consult Baron Cuvier’sOssemens Fossiles, tom. v. To the English reader, the translated abridgement of Cuvier’s "Fossil Remains of the Animal Kingdom," by E. Pidgeon, 1 vol. 8vo. with plates, 1830, will be found a very instructive volume. See also Penny Cyclopædia, Art.Saurians.

Vertebræ of Reptiles.—The bones of the vertebral column of this class of animals present such numerous andimportant modifications in the different orders and families, that reference to the works already cited must be made for satisfactory information on this topic. From the great number of vertebræ in many reptiles, amounting in the individuals of some species to nearly two hundred, these bones are the most abundant fossil relics of these animals to be found in our collections. The vertebræ are commonly detached, and deprived of their processes; the solidcentrum, or body, alone remaining in most examples (as inLign.206,fig.8). Connected series, more or less complete, are occasionally discovered; and the entire column, in connexion with other parts of the skeleton, is preserved in many specimens in the British and other museums.[583]Although, for the reasons previously stated, minute osteological details cannot be attempted in this work, some acquaintance with the elementary characters of the bones composing the spinal column, and of the nomenclature employed to distinguish them, is necessary to guide the student, and even the amateur collector, in their researches. I have, therefore, selected a few specimens from Tilgate Forest in illustration of the elements of Saurian vertebræ, and of the terms by which the different processes are distinguished; the general reader will thus be enabled to comprehend the descriptions of these structures in other works on Palæontology.

[583]SeePetrifactions, pp. 136-352, and pp. 362-387.

Lign. 206. Fossil Vertebræ or Reptiles.Tilgate Forest.The figures are reduced in the proportions specified by the fractions.Fig.1.—Caudal vertebra of an unknown reptile.2.—Chevron bone ofIguanodon: seen in front.3.—Caudal vertebra ofIguanodon, viewed laterally in an oblique direction.3a.——Front view of the same.4.—Caudal vertebra ofIguanodon, without either transverse process or chevron-bone. The letteromarks the deep hollow left by the removal of the transverse process, at the suture of the annular part.5.—Vertebra of Streptospondylus:1/16nat.6.—Lumbar vertebra of Iguanodon, with the neural spine broken off.7.—Vertebra ofStreptospondylus:1/16nat.c.The pair of posterior oblique processes (zygapophyses).8.—The bodies of two dorsal vertebræ of Iguanodon: viewed laterally.The same letters refer to the same parts in the respective figures, with the exception ofcinfig.7.a.The body, or centrum, of the vertebra: the letter denotes the anterior part.b.The annular part, formed of the twoneurapophyses, which contains the spinal cord.c, c.The articular or oblique processes (zygapophyses), which join to the next vertebra in front.d.The spinous process or neural spine of the annular part.e, e.The transverse process of the annular part.f.The chevron-bone, formed of the twohæmapophysesand the hæmal spineg.The double articulating head of the chevron: the passage left by these processes (hæmal arch), seen in the front view, figs. 2 and 3, is for the passage of the large blood-vessels which supply the tail.h.The spine of the chevron-bone; the inferior spinous process, orhæmal spine.i.Denotes the medullary cavity of the annular part.o, o.Mark the sutures which connect the annular part with the body of the vertebra.w, w.Indicate the place of attachment of the chevron-bone.

The bones composing the spine, are not only designed to form a flexible column of support to the trunk, but also to afford protection to the grand nervous chords constituting the spinal marrow, and which extend from the brain to the tail, and give off numerous lateral branches in their course, conferring sensation and motive power to every part of the frame. To effect this purpose, there is attached to the upper or dorsal part of each vertebra a bony ring, called the neural-arch, which is composed of two processes (Lign.206,b.), arising from each side of the body or centrum (Lign.206,a.), and which unite above into a solid piece, termed the spinal process, ox neural spine (Lign.206,d.). On each side of the annular paid there is a process, called the transverse (Lign.206,e, e.), for the attachment of muscles; and in the middle and the posterior dorsal regions of some reptiles, as, for example, in the existing Crocodiles, these processes articulate with the ribs. The vertebræ of the tail have, in addition to the above, an inferior spinous process, termed the chevron-bone (Lign.206,fig.2, andfig.3,f.), which gives support to the inferior layers of the caudal muscles; and, bifurcating at its attachment to the body of the vertebra, leaves a channel for the passage of the large blood-vessels, by which the circulation of the tail is effected.

In the generality of living reptiles (as, for example, in the Crocodile) the bodies of the vertebræ are concave in front, and convex behind; the bones of the spine being united by ball-and-socket joints; but, in most fossil reptiles, both faces are either flat, or more or less concave. In mammalianquadrupeds, the annular part is anchylosed to the vertebral centre; but in reptiles, it is united by suture, although, in old subjects, the connecting line is often obliterated. By reference toLign.206, and its description, the form, arrangement, and connexion of the different vertebral elements, in certain fossil reptiles, may be easily understood. The bones in the vertebral column of the same animal are considerably modified in the several regions of the neck (cervicalvertebræ), back (dorsalandlumbar), and tail (caudal). The cervical are generally of the most complicated structure; and the caudal, the most simple.

From this exposition, the reader will perceive that every vertebra consists of the following essential parts: first, the body, orcentrum; and secondly, the annular part, orneural arch, so named, because it protects the nervous chord; while a caudal vertebra has, in addition, the chevron-bone, called also thehæmal arch, from its affording a passage to the large blood-vessels. The bodies of the vertebræ are in general solid, and consist of the ordinary osseous structure; but in certain fossil vertebræ the centre of the bone is filled with calcareous spar, indicating an irregular medullary cavity, as in the caudal vertebræ of the Ox.[584]

[584]SeePetrifactions, p. 166,note.

TheSacrum, which may be termed the key-stone of the pelvic arch, is formed in existing reptiles by the union of two vertebræ; but in the Iguanodon and the Hylæosaurus the sacrum is composed of six anchylosed vertebræ; in the Megalosaurus probably of but five.[585]

[585]Report, Brit. Assoc. 1841, p. 105, and p. 130.

From the sides of the two anchylosed vertebræ which form the sacrum, strong, short, rib-like processes are given off in those Saurians which occasionally walk on dry land, and these constitute a firm support to the hinder extremities.

In the Crocodiles, the four or five vertebræ preceding thesacrum have no ribs attached to them, and are termedlumbar; in the Lizards, there are but two lumbar vertebræ. A peculiar modification exists in the first caudal vertebra of the adult Gavial and Crocodile; thecentrumisconvexboth in front and behind, as was first demonstrated by me in 1836. SeeWond.p. 419, andPetrif.p. 167. The last of the anchylosed vertebræ forming the sacrum is concave posteriorly; hence the necessity of an anterior ball in the first joint of the tail. (SeeLign.217, p. 676, illustrative ofCrocodilus Hastingsiæ.) The lastcervicalvertebra in the Turtles and Tortoises has a similar construction. This mechanism confers freedom of motion without risk of dislocation.

Ribs.—The Ribs, which are regarded as appendages to the vertebræ, (homologues of the pleurapophyses,) are generally slender and round in the Lizards, and articulate with the spinal column by a single head, supported on a short convex process or tubercle. In Crocodiles only of all existing Reptiles, but in several extinct genera, the proximal end of the rib forms a double articulation, by a distinct head and a tubercle, with the vertebræ in the cervical and anterior dorsal region of the spinal column; in the posterior dorsal region the ribs are attached to the elongated transverse processes of the vertebræ.

As this double articulation of the ribs is invariably associated in existing reptiles with a heart having double ventricles, while the lacertian single-headed ribs are in like manner connected with a heart having but one ventricle, the student will perceive the important physiological inferences that spring from the discovery of a mere fragment of a rib, when interpreted by the profound anatomist.[586]In some fossil reptiles the ribs are flat and very broad; as, for example, in the Hylæosaurus.[587]

[586]See Brit. Assoc. Report, 1841, and Memoirs, Palæont. Soc.[587]In Crocodiles the abdominal region is strengthened by slender ribs (hæmapophyses, Prof. Owen), that are affixed to a ligamentous extension of the cartilaginous sternum, analogous to thelinea albain man; and the Hylæosaurus appears to have possessed a similar development of the costal elements, for I observed many fragments of long, slender, sub-cylindrical rib-like bones whilst chiselling off the stone from this species, and portions of similar bones occur in the stone around the spinal column from Bolney. In the Maidstone Iguanodon there are likewise some long slender bones of this character, which I think must be prolongations of the ordinary dorsal ribs.

[586]See Brit. Assoc. Report, 1841, and Memoirs, Palæont. Soc.

[587]In Crocodiles the abdominal region is strengthened by slender ribs (hæmapophyses, Prof. Owen), that are affixed to a ligamentous extension of the cartilaginous sternum, analogous to thelinea albain man; and the Hylæosaurus appears to have possessed a similar development of the costal elements, for I observed many fragments of long, slender, sub-cylindrical rib-like bones whilst chiselling off the stone from this species, and portions of similar bones occur in the stone around the spinal column from Bolney. In the Maidstone Iguanodon there are likewise some long slender bones of this character, which I think must be prolongations of the ordinary dorsal ribs.

Extremities.—The locomotive extremities are variously constructed, according to the adaptation of the animals to a terrestrial, fluviatile, or marine existence. The bones of the limbs in the extinct colossal terrestrial species much resemble those of our large pachydermata, the Rhinoceros and Hippopotamus. The cylindrical bones of the extremities in the Crocodilians, and other recent reptiles, are solid.i. e.have no cavity filled with marrow; such also is the case in the fossil Enaliosaurians; but the thigh-bones and leg-bones of the Iguanodon, and of other extinct land saurians, h ave a large medullary canal. Our limits will not admit of further osteological details; and we are compelled to omit the description of the bones composing the thoracic and pelvic arches.

Dermal Bones.Ligns.207and208. In many of the reptile tribes, particularly of the Crocodilian or loricated (mailed) group, there are immediately under the external integument or skin a series of osseous scutes, or scutcheons, variously arranged, which serve as supports to the integumental scales and spines. In the gigantic Gavial, that inhabits the Ganges and other rivers of India, and which is remarkably distinguished from the common Crocodile and Alligator by an extremely elongated slender muzzle, the nape of the neck is protected by sixteen or eighteen transverse rows of dermal scutes; and there are likewise six rows which extend down the back. These bones aredeeply corrugated or sculptured on their upper surface; a structure adapted for the firm adhesion of the horny integument.

Detached bones of this character occur in the Purbeck strata; and the first fragments I collected were supposed by me to belong to the soft-skinned turtles (Trionyces); but the subsequent discovery of perfect scutes demonstrated their analogy to the dermal bones of the Gavial, and enabled me to determine their true character.

Lign. 207. Dermal Bone of the Swanage Crocodile:1/3nat.Purbeck.(Goniopholis crassidens.)Fig.1.—The external aspect.2.—The inner surface.a.—The lateral connecting process.

In the splendid specimen of the fossil remains of a Crocodilian reptile (Goniopholis), found at Swanage (Wond.pp. 415; andPetrif.p. 170), there are numerous dermal examples dispersed among the bones, as shown inPetrif.Lign.

One of these is figuredLign.207;fig.1 represents the external surface, which is deeply sculptured by irregularroundish pits or excavations; the under or inner surface,fig.2, is smooth, but marked with very fine striæ, decussating each other at right angles, as in the dermal bones of the Hylæosaurus (Lign.208,fig.1a.). These scutes differ from those of other recent and fossil Crocodilians, in a lateral conical projection, markeda,figs.1, 2,Lign.207, which fits into a depression on the under surface of the opposite angle of the adjoining plate; resembling, in this respect, the scales of the Lepidotus (seeLign.196, p. 605). Numerous hexagonal and pentagonal scutes, articulated together by marginal sutures, also entered into the composition of the osseous dermal cuirass of this reptile, which must, therefore, have possessed a flexible, yet impenetrable, coat of armour, capable of affording protection against the attack of any assailant.

In the Oolite, the dermal bones of other slender-nosed Crocodilians (Teleosaurus) are occasionally met with; the outer surfaces of which are marked with small circular distinct pits; these scutes are thicker and more rectangular than those above described, and slightly overlapped each other laterally; they have no connecting process. In another species one half of the outer surface is smooth, proving that it was covered to that extent by the adjoining scute.[588]

[588]A description of the dermal bones of British fossil reptiles is given in Brit. Assoc. Report for 1841, pp. 70, 79, &c.

Dermal Bones of the Hylæosaurus.—Elliptical and circular dermal scutes, having the under surface flat and the upper convex with a conical tubercle, were first noticed in the specimen of theHylæosaurus, figuredWond.pl. iv.; and I have since discovered similar bones associated with other remains of that extraordinary reptile; reduced figures of two specimens are represented inLign.208,figs.1, 3.

Lign. 208. Dermal Bones of Reptiles.Tilgate Forest.Fig.1, and 3.—Dermal bones of theHylæosaurus:1/3nat.1a.—The under surface of a fragment of a dermal bone, displaying fine spicula, decussating each other at right angles, and indicating a similar structure to that of the Curium, in which the bones were imbedded: nat.1b.—A portion of the same, highly magnified, and viewed by transmitted light.2.—HornofIguanodon:1/6nat.4.—ADorsal Spineof theHylæosaurus; the original is thirteen inches long.

The structure of these bodies is very remarkable; upon closely inspecting the under side, and the surface exposed by a transverse fracture, very minute osseous spicula, decussating each other at right angles, are distinctly seen; as shown inLign.208,fig.1a. Infig.1b, a thin slice of the same, highly magnified, and viewed by transmitted light, displays medullary canals, with very fine lines radiating from them. The peculiar character of this organization consists in the disposition of the straight bony spicula; an appearance which first attracted my attention when developing the original specimen of the Hylæosaurus (seeGeol. S. E.p. 327), and led to the discovery of some perfect examples, whichotherwise would have been destroyed. This structure closely resembles that presented by the ligamentous fibres of thecorium, or skin, and seems to have resulted from an ossified condition of the dermal integument. These bones vary from half an inch to three or four inches in diameter, and were disposed in one or more longitudinal series on each side the spine, diminishing in size as they approach the end of the tail.

Dermal Spines of Hylæosaurus.Lign.208,fig.4.—With the dermal bones above described there are associated in the first discovered specimen of the Hylæosaurus, flat, thin, angular, osseous plates, from three to seventeen inches in length; one of which is figuredLign.208,fig.4. The manner in which they are imbedded in the rocks, in connexion with other parts of the skeleton, is shown Wond. pl. iv. andGeol. S. E.pl. v. These very remarkable processes appeared to me to have formed part of a serrated fringe, which extended along the back of the reptile, analogous to that observable in certain living lizards (Wond.p. 436,Lign.108); and were provisionally described as such in my first memoir on the Hylæosaurus. This conjecture has been substantiated by subsequent discoveries, and the true nature of the large, flat, angular spines, and the conical bones resembling the horn-cones of ruminants, which occur in the Wealden, is now established.[589]

[589]SeeFossils, Brit. Mus.pp. 298, 320.

Horn of Iguanodon.—In this category may be placed the nasal tubercle or horn of a saurian, like that of the Iguana (Lign.208,fig.2;Geol. S. E.pl. iii.), found with the remains of the Iguanodon, and probably belonging to that colossal reptile (Wond.p. 431; andPetrif.p. 298). It is four inches in length and 3.2 inch by 2.1 inch in diameter at the base, which is of an irregular elliptical form. Several smaller specimens have recently been discovered.

Examples of dermal scutes and spines, presenting modifications of form and structure distinct from those above described, have been brought to me from various localities of the Wealden; but, as in no instance a connexion with other parts of the skeleton could be traced, the particular reptiles to which they belonged cannot be ascertained.

We proceed to notice some of the principal genera of Fossil Reptiles, especially of those whose remains occur in the British strata; the arrangement of Professor Owen is adopted for the convenience of reference to the Brit. Assoc. Reports, 1839 and 1841, which should be consulted by the student who would acquire a knowledge of this department of Palæontology. The subject will be considered under the following heads; namely:—

I. Enaliosaurians.—The extinct marine reptiles comprised in this order constitute two genera, which are characterized by essential modifications of osteological structure; they are namedIchthyosaurus(fish-lizard), andPlesiosaurus(akin to a lizard). The general appearance of these beings is so well known, from the splendid collection of their fossil remains in the British Museum, and the numerous specimens in provincial and private collections, and byvarious works, both scientific and popular, in which their structure and physiological relations are fully elucidated, that they must be familiar to every reader.[590]

[590]Bd.vols. i. and ii. contain an admirable exposition of their habits and organization; and Brit. Assoc. Reports, 1839, 1841, elaborate osteological investigations of both genera. A folio volume on these extinct Reptiles, with splendid lithographs, by Thomas Hawkins, Esq., cannot fail to delight the reader by its graphic descriptions and beautiful illustrations. See also the masterly paper on the Ichthyosaur and the Plesiosaur, by the Rev. W. D. Conybeare, in the Geological Transactions, 1st series, vol. v. p. 559,et seq.

ICHTHYOSAURUS. PLESIOSAURUS.

The livingIchthyosaurusmust have borne a resemblance to a Grampus or Porpoise, with four large flippers or paddles and a long tail, having a vertical caudal fin of moderate dimensions; the skin probably being naked and smooth, as in the Cetaceans. ThePlesiosauruspresented a configuration still more extraordinary (Wond.p. 575). With a very small head, it possessed a neck of enormous length, a body of moderate size, with four paddles, resembling those of turtles, and a very short tail. They were both marine, air-breathing, cold-blooded, carnivorous, vertebrate animals; swarming in prodigious numbers during the secondary epochs, and particularly in the seas of the Liassic period (Ly.p. 277, figs. 310, 311). In both genera the construction of the skeleton presents many important variations from all known recent types; and should be carefully investigated by the student, who will find in the Reports of Professor Owen above referred to all the information that can be desired.[591]

[591]The Penny Cyclopædia, Art. Plesiosaurus, contains an able abstract of these Reports; and in theFossils of the British Museum, the student will find a full account of the discovery of the Ichthyosaur and Plesiosaur, and of the deposits in which they are chiefly found, as well as detailed descriptions of the most characteristic structures of the different species.

It will suffice for our present purpose to point out a few important and obvious characters.

Lign. 209. Eye of Ichthyosaurus.1/6nat.Lias.Lyme Regis.Portion of the facial part of the skull of an Ichthyosaurus, showing the position of the nostril, and of the orbit with its circle of bony plates, forming the sclerotic coat of the eye.n.The left nasal aperture.

Lign. 210. Teeth of Ichthyosaurus and Plesiosaurus,nat.Lias.Somersetshire.Fig.1.—Tooth of Plesiosaurus.1a.—Transverse section of the lower part.2.—Tooth of Ichthyosaurus.2a—.Transverse section of the middle of the tooth.3.—Vertical section of part of lower jaw of Ichthyosaurus, with a tooth, illustrative of the mode of dentition.a, a, a.Section of the lower jaw, showing the deep furrowb, in which the teeth are implanted.c.The canal for the dental vessels.d.Foramen for the passage of vessels to the outer integuments.e.Germ of a successional tooth which has occasioned the absorption of the inner portion of the base of the mature tooth,f.

In the Ichthyosaurus, the nasal apertures or openings of the nostrils are not towards the snout, as in the Crocodile, but near the anterior angle of the orbit (seeLign.209), approaching, in this respect, some of the recent lizards. The orbit is very large, and the sclerotic coat or capsule of the eye has in front an annular series of bony plates (Bd.pl. x.figs.1, 3), which often occur in their natural position (Lign.209). This structure is not possessed by fishes, but is analogous to that observable in the eyes of turtles, lizards, and many birds; as for example, in the owl and eagle: it confers on the eye additional power of adaptation and intensity of vision. The muzzle of the Ichthyosaurus is long and pointed; the lower jaw is formed of two branches, united anteriorly through nearly half their length; each branch is composed of six bones, as in the Crocodile and Lizards, but differently arranged than in those reptiles. The teeth are very numerous, amounting to nearly two hundred in some species, and are placed in a single row along the jaws, being implanted in a deep continuous groove (seeBd.pl. xi.). These teeth are of a pointed conical form, longitudinally striated, with an expanded base (Lign.210). The new teeth are developed at the inner side of the base of the old, and grow up and displace them(seeLign.210). The microscopical structure of the teeth of the Ichthyosaurus is beautifully illustrated by Professor Owen (Odontography, p. 275, pl. lxiv.). The tooth consists of a pulp-cavity, surrounded by a body of dentine, which is invested at the base by a thick layer of cement; and at the crown by a coat of enamel, also covered by a pellicle of cement 3 the pulp-cavity, in fully-formed teeth,is more or less occupied by coarse bone. The chief peculiarity of this structure consists in the inflection of the cement into vertical folds at the base of the tooth, by which the marginal portion of the basal dentine is divided into a corresponding number of processes; producing, in a transverse section, the appearance represented inPl. VI. fig. 9. This organization, as we have previously remarked, is similar to that observable in the teeth of theLepidosteus(seep. 616), and of the extinct reptile, calledLabyrinthodon, hereafter to be noticed.

Lign. 211. Vertebra of Ichthyosaurus.1/3nat.Fig.1.—Neural arch and spine.2.—Body or centrum.a.—Socket for the reception of the corresponding process of the neural arch.3.—Vertical section of the centrum.

Lign. 212. The bones composing the Pectoral Arch of Ichthyosaurus.1/8nat.s,sternum.cl,cl,clavicles.sc,sc,scapulæ.c,c,coracoids.

Lign. 213. The bones composing the Pectoral Arch of Plesiosaurus.1/8nat.s,sternum.sc, sc,scapulæ or omoplates.c, c,coracoids.g, g,the glenoid cavities or sockets for the head of the humerus or arm-bones.

The vertebræ; (Bd.pl. xii.; andLign.211), of which there are upwards of one hundred and forty in the individuals of some species, are relatively very short in their antero-posterior diameter (i. e.from front to back); and deeply cupped on each articulating face, as in fishes. The annular part is not united to the body of the vertebra, as in mammals, nor connected by suture, as in Crocodiles, but terminates on each side in a compressed oval base, which fits into corresponding sockets placed on the boundary line of the spinal depression on the body (Lign.210, 2a); thus completing the medullary canal (seeBd.pl. xii.fig.D, E.). Hence the collector may easily recognise the body of an Ichthyosaurian vertebra, by the pits or depressions on the sides of the spinal interspace. The first and second vertebra; are anchylosed together, and have additional subvertebral, wedge-shaped bones, which render this part ofthe column a fixed point of support.[592](Bd.pl. xii. figs. 3, 6.) The form and arrangement of the bones that enter into the composition of the pectoral and pelvic arches, and of the paddles, are exemplified inBd.pl. xii.; and full osteological details are given inBrit. Assoc. Rep.1839, p. 104. The characters of the several bones composing the pectoral arch of the Ichthyosaur will be readily understood from the accompanying illustration. The structure of the pectoral arch of the Plesiosaur is also shown in an accompanying Lignograph, for the sake of comparison. The bones of a fore-paddle of an Ichthyosaurus are represented (Lign.214,fig.1). In some species each paddle consists of nearly one hundred bones. These locomotive extremities are very analogous in their osteological construction to those of the Cetaceans, but they are connected with the trunk by means of the glenoid socket formed by the scapula and coracoid, which are firmly united to the sternum; whereas in the Cetaceans the pectoral fin is only attached to a simple scapula, which is merely suspended in the muscles. This structure, togetherwith the presence of a clavicle in the Ichthyosaurus (seeLign.212), which is wanting in the Cetaceans,, indicates, in the opinion of Professor Owen, that this marine fish-lizard was capable of some degree of locomotion on the land; and that it might have resorted to the shore to deposit its eggs, or, like the Crocodile, to sleep. From the frequent occurrence of a dislocation or abrupt bend of the vertebral series of the tail, at about one-third of its length from the end, supposed to have been produced by the weight of a large fin, during the progress of decomposition, and from the terminal caudal vertebræ being laterally compressed, it is inferred that the Ichthyosaurus had a vertical fin at the extremity of the tail, which would thus be rendered a powerful instrument of progressive motion.[593]From the appearance of theCoprolites, which occur abundantly with the skeletons of these animals, it is obvious that the intestinal canal in the Ichthyosaurus was furnished with spiral valves, as in the Sharks; and the comminuted bones and scales in the coprolites prove that fishes constituted the principal food of these marine reptiles.

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