Gideon Algernon Mantell

FOSSIL REPTILES; COMPRISING THE ENALIOSAURIANS AND CROCODILES.

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"Nous remontons done À un autre Âge du monde; À cet Âge oÙ la terre n’Étoit encore parcourue que par des reptiles a sang froid—oÙ la mer abondoit en ammonites, en bÉlemnites, en tÉrÉbratules, en encrinites, et oÙ tous ces genres, aujourd’hui d’une raretÉ prodigieuse, faisoient le fond de sa population."—Cuvier, Oss. Foss. tom. v. p. 10.

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We advance now to the investigation of the fossil remains of the more highly organized classes of the Vertebrata; the Fishes being the lowest in the scale amongst the beings characterised by an osseous skeleton, with a flexible spinal column, composed of articulated bones, and presenting, in the various classes, orders, genera, and species, numerous modifications of form and structure. The mineralized relics of the vertebrated animals consist, for the most part, of single and displaced bones, or groups of bones and teeth, and the durable portions of the dermal integuments; entire skeletons being of rare occurrence. A knowledge of anatomy and physiology, and access to anatomical and zoological libraries and collections, are therefore indispensable for the cultivation of this most attractive department of PalÆontology. Fortunately for the English student, this branch of the science, which a few years since was but little cultivated in this country,[574] has been greatly advanced, by the liberal support afforded by the British Association of Science to Professor Owen, whose Reports on the British Fossil Reptiles and Mammalia, published in the Transactions of the Association,[575] should be referred to for more precise and detailed information than can be given in these unpretending volumes. Our remarks will be limited to a general notice of the fossil remains of Reptiles, Birds, and Mammals; with descriptions of such characteristic examples, as will serve to illustrate the nature of the specimens that may probably come under the notice of the collector; or which, from their peculiar characters, are objects worthy his special attention.

[574] See Petrif. p. 226, note.[575] Report of the Brit. Assoc. 1839 and 1841; see also Trans. Geol. Soc. 2d ser. vol. v. p. 515 (1838).

The Age of Reptiles.[576]—The announcement by the illustrious founder of PalÆontology, in the quotation prefixed to this chapter, that there was a period when the lakes, rivers, and seas of our planet were peopled by reptiles, and when cold-blooded oviparous quadrupeds, of appalling magnitude, were the principal inhabitants of the dry lands, was a proposition so novel and startling, as to require the prestige of the name of Cuvier to obtain for it any degree of credence, even with those who were prepared to admit that a universal deluge could not account for the physical changes, which the crust of the earth had evidently undergone. Subsequent observations and discoveries have, however, fully confirmed the truth of this induction, and the "Age of Reptiles" is no longer considered fabulous.

[576] "The Age of Reptiles" was the title given by the author to a popular summary of the evidence bearing on this question: it was published in the Edinburgh Philosophical Journal, 1831. This name is now generally employed to designate the geological epochs characterised by the predominance of oviparous quadrupeds; namely, from the Permian to the Chalk, inclusive.

In some of the ancient fossiliferous deposits,[577] indications of the existence of Reptiles are visible, in the indelible markings left by their footsteps on the muddy banks of rivers, and on the wet sands of the sea-shores, now in the state of layers of marl and sandstone. Here and there in the Devonian,[578] Carboniferous, and New Red formations, teeth and bones are found, presenting unequivocal proofs of the presence of extinct forms of cold-blooded oviparous quadrupeds. As we ascend in the secondary formations, we are suddenly surrounded by innumerable marine and terrestrial reptiles, belonging to species and genera, and even orders, of which no living representatives are known. Throughout the Liassic, Oolitic, Wealden, and Cretaceous epochs, the class of Reptiles was at its fullest development. In the Tertiary periods which succeeded, the Reptiles approach the recent types, and their relics are found intermingled with the bones of mammiferous quadrupeds; thus indicating the commencement of the present condition and relations of the animal kingdom. Referring the reader to Bd. p. 165, and Wond. pp. 409-444 and 567-588, for a more comprehensive view of this subject, we advance to the examination of some of the fossil genera and species; and we propose, in the first place, to explain a few essential characters of form and structure observable in those durable parts of the skeletons which are most frequently met with in a fossil state; namely, the teeth, jaws, vertebrÆ, &c., and the osseous appendages of the dermal system.

[577] Devonian rooks of Elgin, North Britain; and the Lower Carboniferous of Pottsville, Pennsylvania.[578] The most ancient Reptile hitherto discovered is the Telerpeton Elginense, from the Old Red of Scotland, which will be described in the sequel.

The animals comprehended in the Class of Reptilia constitute, according to Prof. Owen’s arrangement, eight principal groups, or Orders, as follow:—

The Batrachia; Frogs: the body naked, with only rudimentary ribs; and with two or four feet. Most of these reptiles breathe by branchiÆ or gills in their young state, and by lungs in the adult (as for example the Frog); in some (the perenni-branchiata), the branchiÆ are persistent through life.	}	Having a tripartite heart (i. e. with two auricles and one ventricle), and simple transverse processes to the cervical and anterior dorsal vertebrÆ.
The Ophidia; Serpents: the body destitute of feet.
The Lacertia or Sauria; Lizards: the body supported by four or two feet, and covered with scales.
The Chelonia; Tortoises: the body supported by four feet or paddles, and enveloped in two osseous bucklers, composed of the expanded bones of the sternum and thorax.
The Enaliosauria; Sea-saurians (extinct): body furnished with four paddles, and destitute of scaly covering.	}	Having a quadripartite heart (i. e. with two auricles and two ventricles), and double transverse processes to the cervical and anterior dorsal vertebrÆ.
The Pterosauria; Wing-saurians (extinct): body supported on four feet, the outer finger of each fore-foot greatly lengthened, and forming a support for the wing.
The Crocodilia; Crocodiles: body supported on four partially webbed feet, and encased with an armour of bony plates or scutes.
The Deinosauria; Great-saurians (extinct): body supported on four feet.

Teeth of Reptiles.—The teeth of the animals of this class exhibit considerable diversity of form, but the characteristic type is that of a conical, pointed tooth, with a simple root or fang; for, in no reptile does the base of the tooth terminate in more than one fang, and this is never branched. "Any fossil, therefore, which exhibits a tooth implanted by two fangs in a double socket, must be mammiferous, since the socketed teeth of reptiles have but a single fang; and the only fishes’ teeth which approach such a tooth in form, are those of a bifurcate base, belonging to certain sharks." (Owen.)

These dental organs are only fitted for seizing and retaining the prey or food; for no living reptiles have the power of performing mastication. In the Crocodiles the tooth has a cylindrical shank, with a conical, longitudinally striated, enamelled crown, having a ridge on each side (Pl. VI. fig. 5). In the Labyrinthodon (a fossil reptile), the cone is more curved and pointed (Pl. VI. fig. 3); in the HylÆosaurus, the shank is cylindrical, and the crown expanded and lanceolate, with blunt margins (Pl. VI. fig. 6); in the Megalosaurus, the tooth is laterally compressed, trenchant, and slightly inclined backwards like a sabre, with serrated edges (Pl. VI. fig. 7); in the Iguanodon, the shank is cylindrical, and the crown of a prismatic form, greatly expanded, with broad denticulated edges, and longitudinal ridges on one side (Pl. VI. fig. 4, and Ligns. 221, 223). In the Serpents, the teeth are very long and pointed; in the Crocodiles and Lizards, may be seen every modification of the conical form, down to a mere hemispherical tubercle or plate. In the fossil Dicynodon, to be hereafter described, the dental system consists of but two tusks or canine teeth, like those of the Walrus, implanted in the upper jaw. The Turtles and some fossil Lacertians are edentulous, i. e. destitute of teeth; their dental organs consisting of the horny trenchant sheaths with which the jaws are covered.

The teeth are very numerous in reptiles; the individuals of some species have more than two hundred. In some genera, they are implanted on the jaws alone; in many, they occupy the palatine, vomerine, and other bones composing the vault of the mouth, as in certain fishes. The teeth are generally anchylosed to the bone; but in some genera they are implanted in distinct sockets, as in the Crocodile and Plesiosaurus; in others, as in the Ichthyosaurus, they are arranged in a deep furrow, and retained only by the integuments; in some, they are supported upon an elevated osseous base. In the Labyrinthodonts, and in the greater part of the Serpent tribes, the tooth is implanted by the base in a shallow socket, with which it is confluent.

In most of the Lacertians, or true Lizards, the attachment of the teeth presents a peculiar modification, of which the lower jaw of the Iguana, Lign. 205, p. 649, affords a good illustration. The teeth are not placed in sockets, but are attached by the shank to an alveolar plate, or parapet, that extends along the margin of the jaw, as shown in figs. 1 and 3; the crowns of the teeth project above this plate, as seen in figs. 2 and 4. From the anchylosis of the teeth to the side of the jaw, the Lizards possessing this dental structure are termed Pleurodonts.[579]

[579] The Pleurodonts are those lizards in which the teeth are anchylosed to the side of the dentary bone; Acrodonts, those with the teeth fixed to the upper margin or ridge of the jaw-bone; Thecodonts, those having the teeth implanted, either loosely, or anchylosed to the walls of their sockets. Lacertians are also said to be Pleodont (having solid teeth), or Coelodont (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]

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. 4b and 4c. But another modification is that to which allusion was made when describing the teeth of the Lepidosteus (see p. 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.

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 various sizes placed one upon another; for in reptiles the production of new teeth is unlimited. But in the Pleurodont lizards, 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 in Lign. 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, the dentary bone, supporting the teeth; b, the splenial or opercular; c, the coronoid or complementary; d, the sur-angular; e, the angular; f, the articular, 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 in Lign. 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 the dentary bone remains, indications may be obtained of the family, and perhaps genus, to which it belonged.[582]

VertebrÆ of Reptiles.—The bones of the vertebral column of this class of animals present such numerous and important 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 solid centrum, or body, alone remaining in most examples (as in Lign. 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.

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, and fig. 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 mammalian quadrupeds, 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 to Lign. 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 (cervical vertebrÆ), back (dorsal and lumbar), 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, or centrum; and secondly, the annular part, or neural arch, so named, because it protects the nervous chord; while a caudal vertebra has, in addition, the chevron-bone, called also the hÆmal 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]

The Sacrum, 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]

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 the sacrum have no ribs attached to them, and are termed lumbar; in the Lizards, there are but two lumbar vertebrÆ. A peculiar modification exists in the first caudal vertebra of the adult Gavial and Crocodile; the centrum is convex both in front and behind, as was first demonstrated by me in 1836. See Wond. p. 419, and Petrif. 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. (See Lign. 217, p. 676, illustrative of Crocodilus HastingsiÆ.) The last cervical vertebra 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]

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. 207 and 208. 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 are deeply 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.

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

One of these is figured Lign. 207; fig. 1 represents the external surface, which is deeply sculptured by irregular roundish 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, marked a, 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 (see Lign. 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]

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 the HylÆosaurus, figured Wond. pl. iv.; and I have since discovered similar bones associated with other remains of that extraordinary reptile; reduced figures of two specimens are represented in Lign. 208, figs. 1, 3.

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 in Lign. 208, fig. 1a. In fig. 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 (see Geol. S. E. p. 327), and led to the discovery of some perfect examples, which otherwise would have been destroyed. This structure closely resembles that presented by the ligamentous fibres of the corium, 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 figured Lign. 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. and Geol. 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]

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; and Petrif. 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 named Ichthyosaurus (fish-lizard), and Plesiosaurus (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 by various works, both scientific and popular, in which their structure and physiological relations are fully elucidated, that they must be familiar to every reader.[590]

The living Ichthyosaurus must 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. The Plesiosaurus presented 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]

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

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 (see Lign. 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 (see Bd. 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 (see Lign. 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 in Pl. VI. fig. 9. This organization, as we have previously remarked, is similar to that observable in the teeth of the Lepidosteus (see p. 616), and of the extinct reptile, called Labyrinthodon, hereafter to be noticed.

The vertebrÆ; (Bd. pl. xii.; and Lign. 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 (see Bd. 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 of the 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 in Bd. pl. xii.; and full osteological details are given in Brit. 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, together with the presence of a clavicle in the Ichthyosaurus (see Lign. 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 the Coprolites, 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.

[592] This structure was first demonstrated by Sir Philip Egerton. See Geol. Trans. 2d series, vol. v. p. 187, pl. xiv.[593] Geol. Trans. 2d ser. vol. v. p. 511, pl. xlii.