The horizontality of the tail-fin of the whale tribe is essentially connected with their necessities as warm-blooded animals breathing atmospheric air; without this means of displacing a mass of water in the vertical direction, the head of the whale could not be brought with the required rapidity to the surface to respire; but the Ichthyosaurs, not being warm-blooded, or quick breathers, would not need to bring their head to the surface so frequently, or so rapidly, as the whale; and, moreover, a compensation for the want of horizontality of their tail-fin was provided by the addition of a pair of hind-paddles, which are not present in the whale tribe. The vertical fin was a more efficient organ in the rapid cleaving of the liquid element, when the Ichthyosaurs were in pursuit of their prey, or escaping from an enemy.
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, or 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, as it is called, 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 Ornithorhynchus, Platypus, and Duck-mole. The Ichthyosaurs, when so visiting the shore, either for sleep, or procreation, would lie, or crawl prostrate, or with the belly resting or dragging on the ground.
The most extraordinary feature of the head was the enormous 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 (cavity for the eye), 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 eyeball 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,[46] “that the enormous eye, of which they formed the front, was an optical instrument of varied and prodigious power, enabling the Ichthyosaurus to descry its prey at great or little distances, in the obscurity of night, and in the depths of the sea.”
[46] Op. cit., p. 174.
Of no extinct reptile 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: scale-less, moreover, and covered by a smooth, or finely wrinkled skin analogous to that of the whale tribe.
The mouth was wide, and the jaws long, and armed with numerous pointed teeth, indicative of a predatory and carnivorous 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 specimens, 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 impression of the structure of the internal surface of the intestine of the great predatory sea-lizard. These digested masses are called “coprolites.”
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.
No. 12.—Ichthyosaurus platyodon.
This most gigantic species, so called on account of the crown of the tooth being more flattened than in other species, and having sharp edges, as well as a sharp point, was first discovered in the has of Lyme Regis, in Dorsetshire. Fossil remains now in the British Museum, and in the museum of the Geological Society, fully bear out the dimensions exhibited by the restoration of the animal as seen basking on the shore between the two specimens of Long-necked Plesiosaurs. The head of this species is relatively larger in proportion to the trunk, than in the Ichthyosaurus communis or Ichthyosaurus tenuirostris: the lower jaw is remarkably massive and powerful, and projects backwards beyond the joint, as far as it does in the crocodile. In the skull of an individual of this species, preserved in the apartments of the Geological Society of London, the cavity for the eye, or orbit, measures, in its long diameter, fourteen inches. The fore and hind paddles are large and of equal size.