SQUAMATA 



i6s 



of a sufficient size to cope with the fierce enemies of the open seas. 

 We know practically nothing of the inhabitants of the lakes and 

 rivers during all the time in which the mosasaurs existed; and this 

 perhaps is the real reason why we have never yet found a specimen 

 of a young mosasaur. 



We have seen that many skeletons of ichthyosaurs are found 

 entire, and but little disturbed in position, suggesting, if not proving, 

 that the animals as a rule lived and died far out in the deep seas, 

 away from the disturbing effects of currents of water on their 

 decaying bodies. Among the thousands collected, the great 

 majority of the specimens of mosasaurs consist of a few bones 

 or a part of the skeleton only. Moreover, nearly all specimens 



Fig. 79. — Head of Tylosauriis 



show the disturbing effects of currents of water; and the bones 

 are usually associated with those of turtles, birds, and flying reptiles, 

 which probably did not often venture far from the shores; all of 

 which goes to prove that the mosasaurs in general lived in the com- 

 paratively shallow waters of the seas, and not far from the shores. 

 That some were excellent divers, descending probably many 

 fathoms deep in the water, is certain, because of the extraordinary 

 protective structures of the eyes and ears. 



But the various kinds of mosasaurs differed not a little in their 

 habits. Some, like Mosasaurus and Clidasks, were doubtless 

 chiefly surface swimmers, as is evidenced by their better ossified 

 bones, firmer articulation, and the presence of the additional 



