ICHTHYOSAURUS. 137 



as well as of Turtles, Tortoises, and Lizards; and in a less 

 degree in Crocodiles. (PI. 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 convexity 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 Ichthyosauri have of course entirely perished ; 

 but the preservation of this curiously constructed hoop of 

 bony plates, shows 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 ; it also tends to associate the 

 animal, in which it existed, with the family of Lizards, and 

 exclude it from that of fishes.* 



A farther advantage resulting from this curious appara- 



assist to maintain the prominent position of the front of the eye, which is so 

 remarkable in birds. In Owls, wliose nocturnal liabits render distant vision 

 impossible, Mr. Yarrel observes, that the bony circle (PI. 10, Fig. 4,) is con- 

 cave, and elongated forwards, so that the front of the eye is placed at the 

 end of a long tube, and thus projects beyond tlie loose and downy feathers of 

 the head; he adds; "The extent of vision enjoyed by the Falcons is proba- 

 bly denied to the Owls, but their more spherical lens and corresponding 

 cornea give them an intensity better suited to the opacity of the medium in 

 whiclj^they are required to exercise this power. They may be compared 

 to a person nearsighted, who sees objects with superior magnitude and bril- 

 liancy when within the prescribed limits of his natural powers of vision, from 

 the increased 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 ossifica- 

 tion of the exterior capsule; but in fishes, this ossification is usually simple, 

 though carried to a different extent in difiTerent species; and the bone is 

 never divided transversely into many plates, as in Lizards, and Birds; thesa 

 capsules of the eye are often preserved in the heads of fossil fishes: they 

 j^bound in the London clay; and occasionally occur in chalk. 



12* 



