138 MARINE SAURIANS. 



tus of bony plates, was to give strength to the surface of so 

 large an eye-ball, enabling it the better to resist the pressure 

 of deep water, to which it must often have been exposed; it 

 would also have protected this important organ from injury 

 by the waves of the sea, to which an eye, sometimes larger 

 than a man's head, must frequently have been subject, when 

 the nose was brought to the surface, for the necessary pur- 

 pose of breathing air : the position of the nostrils, close to 

 the anterior angle of the eye, rendered it impossible for the 

 Ichthyosaurus to breathe without raising its eye to the sur- 

 face of the water. 



Jaws, 



The Jaws of the Ichthyosauri, like those of Crocodiles 

 and Lizards, which are all more or less elongated into pro- 

 jecting beaks, are composed of many thin plates, so arranged 

 as to combine strength with elasticity and lightness, in a 

 greater degree than could have been effected by single bones, 

 like those in the jaws of Mammalia. It is obvious that an 

 under jaw so slender, and so much elongated as that of a 

 Crocodile or Ichthyosaurus, and employed in seizing and re- 

 taining the large and powerful animals which formed their 

 prey, would have been comparatively weak and liable to 

 fracture if composed of a single bone. Each side of the 

 lower jaw was therefore made up of six separate pieces, set 

 together in a manner that will be best understood by refer- 

 ence to the Figures in PI. 1 1 .* 



* These figures are selected from various plates by Mr. Conybeare and 

 Mr. De la Beche. Fig. 1 is a restoration of the entire bead of an Iclitiiy- 

 osaurus, in which each component bone is designated by the letters ap- 

 propriated by Cuvier to the equivalent bones in the head of the Croco- 

 dile. In the lower jaw, u, marks the dental bone; v, the angular bone; 

 X, superangidar or coronoid ; y, articular bone ; z, complimentary; 4-, 

 cperculai'. Fig. 2, is part of an under jaw of an Ichthyosaurus, showing 



