136 MARINE SAURIANS. 



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. 



Eyes. 



The enormous magnitude of the eye of the Ichthyosaurus 

 (PI. 10, Fig. 1, 2,) is among the most remarkable pecuUari- 

 ties 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 proper- 

 ties. 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 

 (PI. 10, Fig. 3.) This compound circle of bony plates, does 

 not occur in fishes ; but is found in the eyes of many birds.f 



* In PI. 11. Fig'. A, shows the manner in which the older tooth in the Croco- 

 dile becomes absorbed, by pressure of a younger tooth rising within the ca- 

 vity of its hollow base. Fig. c, represents a transverse section of the left 

 side of the lower jaw of an Ichthyosaurus, showing two teeth in their natu- 

 ral place, within the farrows of tlic jaw ; the younger tooth, by lateral pres- 

 sure, 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 Ichthyosau- 

 rus, 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. 



t The bony sclerotic of the Iclithyosaurus approaclies to the form of 

 the bony circle in the eye of tiie Golden Eagle (PI. 10, Fig. 5;) one of its 

 uses in each case being to vary the sphere of distinct vision, in order ta 

 descry their prey at long or short distances. These bony plates alsa 



