ICHTHYOPTERYGIA. 227 



to find in front of the orbit in fossil skulls, a circular series of 

 petrified thin bony plates, ranged round a central aperture, 

 where the pupil of the eye was placed. The eyes of many 

 fishes are defended by a bony covering consisting of two 

 pieces ; but a compound circle of overlapping plates is now 

 found only in the eyes of turtles, tortoises, lizards, and birds. 

 This curious apparatus of bony plates would aid in protecting 

 the eye-ball from the waves of the sea when the Ichthyosaurus 

 rose to the surface, and from the pressure of the dense element 

 when it dived to great depths; and they shew, writes Dr. 

 Buckland,* " 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 dis- 

 tances, in the obscurity of night, and in the depths of the sea." 



In the Ichthyosaurus communis there are seventeen sclerotic 

 plates forming the fore part of the eyeball. In a well-pre- 

 served example in the British Museum, the pupillary or cor- 

 neal vacuity, as bounded by those plates, is of a full oval 

 form, \\ inch in long diameter, the length of the plates (or 

 breadth of the frame) being from 8 to 10 lines. In the same 

 skull the long diameter of the orbit is 4 inches. The deep 

 position of the sclerotic circle in this cavity shewed how they 

 had sunk, by pressure of the external mud, as the eyeball 

 became collapsed by escape of the humours in decomposition. 



Of no extinct species are the materials for a complete and 

 exact restoration more abundant and satisfactory than of the 

 Ichthyosaurus ; they plainly shew that its general external 

 figure must have been that of a huge predatory abdominal fish, 

 with a longer tail and a smaller tail fin ; scaleless, moreover, 

 and covered by a smooth or finely wrinkled skin, analogous 

 to that of the whale tribe, t 



* Bridgeicater Treatise, Ed. 1858, vol. i., p. 171. 



t Trans. Geol. 8oc., 2d Ser., vol. v. The specimen there described shews . 

 the shape of the fin as in the living animal. 



