Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/140

 136 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.

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.

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,