154 WATER REPTILES OF THE PAST AND PRESENT 



use like that of the modern aquatic monitors, as mentioned on a 

 preceding page. The eyes were of moderate size, those of the less 

 purely aquatic forms being directed more laterally than those of 

 species of more distinctly diving habits. They were protected 

 by a stout ring of bony plates, as were the eyes of all truly aquatic 

 reptiles of the past. The ears, also, in most if not all mosasaurs, 

 had a thick cartilaginous ear-drum in place of a simple membrane, 

 evidently, as Dollo has shown, for better protection under undue 

 pressure of the water in deep diving. 



As in all other lizards, the bones with which the lower jaws 

 articulate, the quadrates, were loosely attached at the upper end, 

 permitting great freedom of movement in all directions, more even 

 than the land lizards have. The lower jaws were long and powerful, 

 armed with a single row of teeth on each side, from sixteen to 



pa. 



Fig. 71. — Clidastes, inner side of right mandible: ang, angular; art, articular; 

 cor, coronoid; pa, prearticular; sur, surangular. 



eighteen in number. Just back of the teeth, a little beyond the 

 middle, each mandible has a remarkable joint, quite unknown in 

 land lizards, though a trace of it is found in the monitors, per- 

 mitting much movement between the front and back parts, both 

 laterally and vertically, though chiefly in the former direction. 

 Furthermore, as in land snakes but not as in land lizards, the 

 front ends of the two sides of the jaws were somewhat loosely 

 attached to each other by ligaments. This looseness of the two 

 sides of the jaws, not only in front but also behind, together with 

 the joint in each, was of the greatest use in swallowing prey, as 

 will be explained farther on. 



As in most other aquatic reptiles, the neck was short and strong, 

 the vertebrae being less in number than in most other lizards. 

 The trunk was long and slender, more especially so in the surface- 

 swimming kinds, with from twenty-two to thirty-four vertebrae. 



