SQUAMATA 



153 



scarcely eight feet in length. There are now known at the present 

 time seven or eight genera of three distinct tj^es, all belonging 

 to one family, the Mosasauridae, including about twenty-five 

 known species. While a few of the genera are widely distributed 

 over the earth, the species are all of restricted range, indicating, 

 perhaps, non-migratory habits. 



The adaptation of the mosasaurs to an aquatic life was very 

 complete, though perhaps not so complete as was that of the 

 ichthyosaurs. The skull is flattened, narrow, and more or less 

 elongate, but large in proportion to the re- 

 mainder of the skeleton — nearly one-sixth of 

 the entire length; that relative size doubtless 

 is indicative of very predaceous and pugna- 

 cious habits. The teeth in the typical forms 

 are numerous, strong, and sharp, conical in 

 shape, and recurved. Not only are there 

 numerous teeth in both the upper and lower 

 jaws, but there are also two rows of strong 

 teeth implanted in the back part of the 

 palate, upon bones called pterygoids, the use 

 of which will be understood later. The teeth 

 were inserted on large, tumid, bony bases, 

 rather loosely attached in shallow pits or 

 alveoli, unlike the teeth of all modern lizards. 

 Such a mode of attachment of the teeth 

 doubtless had some relation to the habits of 

 the animals concerning which we are not 

 quite clear. They were easily dislodged, and, in consequence, of 

 very unequal size, some full grown, some small, and others just 

 appearing above the surface of the gums in the living animals. 

 The frequent loss of teeth and their constant and easy replace- 

 ment by new ones is a peculiarity of predaceous reptiles, thereby 

 insuring their best functional use. 



The external nostrils, of large size, were situated at a consider- 

 able distance back of the end of the snout, but not nearly so far 

 back or so near the eyes as were the nostrils of the ichthyosaurs, 

 plesiosaurs, and phytosaurs. Their size and position suggest a 



Fig. 70.— Tooth of 

 Tylosaurus, two-thirds 

 natural size. 



