on one side, it causes first a partial absorption of the base, and finally a total removal of the body of the older tooth, which it is destined to replace.[1]
As the predaceous habits of the Ichthyosauri exposed them, like modern Crocodiles, to frequent loss of their teeth, an abundant provision has in each case been made for their continual renewal.
Eyes.
The enormous magnitude of the eye of the Ichthyosaurus (Pl. 10, Fig. I, 2,) is among the most remarkable peculiarities in the structure of this animal. From the quantity of light admitted in consequence of its prodigious size, it must have possessed very great powers of vision; we have also evidence that it had both microscopic and telescopic properties. We find on the front of the orbital cavity in which this eye was lodged, a circular series of petrified thin bony plates, ranged around a central aperture, where once was placed the pupil; the form and thickness of each of these plates very much resembles that of the scales of an artichoke (Pl. 10, Fig. 3.) This compound circle of bony plates, does not occur in fishes; but is found in the eyes of many birds,[2]
- ↑ In Pl. II, Fig. A, shows the manner in which the older tooth in the Crocodile becomes absorbed, by pressure of a younger tooth rising within the cavity of its hollow base. Fig. e, represents a transverse section of the left side of the lower jaw of an Ichthyosaurus, showing two teeth in their natural place, within the furrows of the jaw; the younger tooth, by lateral pressure, has caused absorption of the inside portion of the base of the older tooth. Fig, B, represents a transverse section of the entire snout of an Ichthyosaurus, in which the lower jaw exhibits on both sides, a small tooth (a,) which has caused partial absorption of the base of the larger tooth, (c.) In the upper jaw, the bases of two large teeth (d, d,) are seen in their respective furrows.
- ↑ The bony sclerotic of the Ichthyosaurus approaches to the form of the bony circle in the eye of the Golden Eagle (Pl. 10, Fig. 5;) one of its uses in each case being to vary the sphere of distinct vision, in order to descry their prey at long or short distances. These bony plates also
