| 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.
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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.
Lign. 211. Vertebra of Ichthyosaurus. 1/3 nat.
| Fig. | 1.— | Neural arch and spine. | |
| 2.— | Body or centrum.
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| 3.— | Vertical section of the centrum. |
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.