174 MARINE SAURIANS. 
Tortoises, and Lizards ; and in a less degree in 
Crocodiles. (PL 10, Figs. 4. 5. 6.) 
In living animals these bony plates are fixed 
in the exterior or sclerotic coat of the eye, and 
vary its scope of action, by altering the con- 
vexity of the cornea : by their retraction they 
press forward the front of the eye and convert 
it into a microscope ; in resuming their position, 
when the eye is at rest, they convert it into a 
telescope. The soft parts of the eyes of the Ich- 
thyosauri have of course entirely perished ; but 
the jDreservation of this curiously constructed hoop 
of bony plates, shows that the enormous eye, of 
which they formed the front, w^as an optical in- 
strument 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 ; it also tends to associate 
the animal, in which it existed, with the family 
of Lizards, and exclude it from that of fishes.* 
power. They may be compared to a person near-sighted, who 
sees objects with superior magnitude and brilliancy when within 
the prescribed limits of his natural powers of vision, from the in- 
creased angle these objects subtend." Yarrel on the Anatomy of 
Birds of Prey, Zool. Journal, v. 3, p. 188. 
* There are analogous contrivances for the purpose of resisting 
pressure, and maintaining the form of the eye in fishes, by the 
partial or total ossification of the exterior capsule ; but in fishes, 
this ossification is usually simple, though carried to a diflferent 
extent in different species ; and the bone is never divided trans- 
versely into many plates, as in Lizards and Birds ; these capsules 
of the eye are often preserved in the heads of fossil fishes : they 
abound in the London clay ; and occasionally occur in chalk. 
