In the Ichthyosaurus communis there are seventeen sclerotic plates forming the fore part of the eyeball. In a well-preserved example, the pupillary or corneal vacuity, as bounded by those plates, is of a full oval form, 1 inch in long diameter, the length of the plates (or breadth of the frame) being from 8 to 10 lines. In the same skull the long diameter of the orbit is 4 inches. The deep position of the sclerotic circle in this cavity showed how they had sunk, by pressure of the external mud, as the eyeball became collapsed by escape of the humours in decomposition.
Whenever the antecedent forms of an extinct genus of any class are known, the characters of such genus should be com pared with those of its predecessors in such class, rather than with its successors or with existing forms, in order to gain an insight into its true affinities.
We derive a truer conception of the affinities of the Ichthyosaurus by comparison with the Labyrinthodonts and other triassic reptiles, as we do of the Plesiosaurus by com parison with the muschelkalk Sauropterygia, than of either by comparison with modern Lacertians and Crocodilians. It is commonly said that the Ichthyo- or the Plesiosaurus resembles more the lizards in such and such characters, and in a less degree the crocodiles, as in such a character. The truer expression would be that the lizards, which are the predo minating form of Saurians at the present day, have retained more of the osteological type of the triassic and oolitic reptiles, and that the crocodiles deviate further from them or exhibit a more modified or specialized structure. The posterior position of the nostrils, the small size and position of the palato-pterygoid foramen, are marks of affinity to Plesiosaurus, in common with which genus the cranial structure of the Ichthyosaurus exhibits a majority of lacertian characters.
In comparing the jaws of the Ichthyosaurus tenuirostris with those of the gangetic Gharrial, an equal degree of strength and of alveolar border for teeth result from two very different proportions in which the maxillary and premaxillary bones are combined together to form the upper jaw. The prolonga tion of the snout has evidently no relation to this difference ; and we are accordingly led to look for some other explanation of the disproportionate development of the premaxillaries in the Ichthyosaurus. It appears to me to give additional proof of the collective tendency of the affinities of the Ichthyosaurus to the lacertian type of structure. The backward or antorbital position of the nostrils, like that in whales, is related to their marine existence. But in the Lacertians in which the nostrils extend to the fore part of the head, their anterior boundaries are formed by the premaxillaries : it appears, therefore, to be in conformity with the lacertian affinities of the Ichthyosaur that the premaxillaries should still enter into the same relation with the nostrils, although this involves an extent of anterior development proportionate to the length of the jaws, the forward production of which sharp-toothed instruments fitted them, as in the modern dolphins, for the prehension of agile fishes.
That the Ichthyosaurs occasionally sought the shores, crawled on the strand, and basked in the sunshine, may be inferred from the bony structure connected with their fore fins, which does not exist in any porpoise, dolphin, grampus, and whale ; and for want of which, chiefly, those warm-blooded, air-breathing, marine animals are so helpless when left high and dry on the sands. The structure in question in the
Ichthyosaur is a strong osseous arch, inverted and spanning across beneath the chest from one shoulder-joint to the other ; and what is most remarkable in the structure of this "scapular" arch is, that it closely resembles, in the number, shape, and disposition of its bones, the same part in the singular aquatic mammalian quadruped of Australia, called Ornithorynckus, and Platypus, or duck-mole. The Ichthyosaur, when so visiting the shore either for sleep or procreation, would lie or crawl prostrate, or with its belly resting or dragging on the ground.
The most extraordinary feature of the head was the enor mous magnitude of the eye : and from the quantity of light admitted by the expanded pupil, it must have possessed great powers of vision, especially in the dusk. It is not uncommon to find in front of the orbit in fossil skulls, a circular series of petrified thin bony plates, ranged round a central aperture, where the pupil of the eye was placed. The eyes of many fishes are defended by a bony covering consisting of two pieces ; but a compound circle of overlapping plates is now found only in the eyes of turtles, tortoises, lizards, and birds. This curious apparatus of bony plates would aid in protecting the eye-ball from the waves of the sea when the Ichthyosaurus rose to the surface, and from the pressure of the dense element when it dived to great depths ; and they show, writes Dr. Buckland (Bridgewater Treatise), "that the enormous eye of which they formed the front, was an optical instrument of varied and prodigious power, enabling the Ich,lh,yosaurus to descry its prey at great or little distances, in the obscurity of night, and in the depths of the sea." Of no extinct species are the materials for a complete and exact restoration more abundant and satisfactory than of the Ichthyosaurus; they plainly show that its general external figure must have been that of a huge predatory abdominal fish, with a longer tail and a smaller tail fin ; scaleless, more over, and covered by a smooth or finely wrinkled skin, analo gous to that of the whale tribe.
The month was wide, and the jaws long, and armed with numerous pointed teeth, indicative of a predatory and carni vorous nature in all the species ; but these differed from one another in regard to the relative strength of the jaws, and the relative size and length of the teeth.
Masses of masticated bones and scales of extinct fishes, that lived in the same seas and at the same period as the Ichthyosaurus, have been found under the ribs of fossil speci mens, in the situation where the stomach of the animal was placed ; smaller, harder, and more digested masses, containing also fish-bones and scales, have been found, bearing the impres sion of the structure of the internal surface of the intestine of the great predatory sea-lizard. One of these " coprolites n is figured beneath the skeleton in fig. 68.
In tracing the evidences of creative power from the earlier to the later formations of the earth's crust, remains of the Ichthyosaurus are first found in the lower lias, and occur more or less abundantly through all the superincumbent secondary strata up to, and inclusive of, the chalk formations. They are most numerous in the lias and oolite, and the largest and most characteristic species have been found in these formations.
More than thirty species of Ichthyosaurus are known to the writer, many of which have been described or defined.