THALATTOSAURIA 



173 



openings on each side in the roof of the skull, as in the modern 

 tuatera. While this character may seem trivial, it is really one of 

 the most important in the reptilian anatomy in determining the 

 relationship and classification of reptiles. The teeth are conical 

 and pointed in the front end of the upper and lower jaws, but 

 farther back they are rounded, rugose, and obtuse, and could 

 have been used only for crushing hard objects, like mollusks, 

 crustaceans, etc. (Fig. 82). And not only was there a row of such 

 teeth on each jaw (only partly seen in the figure), but similar 

 teeth covered a large part of the palate. And 

 the lower jaws, it is seen, are rather massive. 

 The vertebrae were, of course, of the more 

 primitive kind, that is, with the ends con- 

 cave, both in front and behind. It would 

 have been strange indeed were they of any 

 other kind, since reptiles with ball-and-socket 

 joints to the vertebrae, that is, concave on 

 one end and convex on the other, as in 

 nearly all living reptiles, did not come into 

 existence till long after the thalattosaurs had 

 disappeared from geological history; and it 

 is also a curious fact that such vertebrae 

 appear to have originated only among animals 

 crawling on land, so that they would not 

 have been a character acquired by the thalattosaurs after descend- 

 ing into the water. It will be seen from the figure of a dorsal 

 vertebra that the rib was attached by a single articular surface, 

 almost exclusively to the body of the vertebra, quite like those 

 of all lizards, snakes, and mosasaurs, and unlike those of other 

 reptiles. This too may seem to be a trivial character to prove 

 relationships with the lizards, but it is a curious fact that no two 

 animals having different kinds of ribs are closely related to each 

 other. Possibly, however, this looser mode of attachment of the 

 ribs in the thalattosaurs was one of their peculiar adaptations to a 

 water life, and may not have been derived from their land ancestors. 

 Of the limbs, only a few bones are known, but these are very 

 instructive. The arm bones, as shown in Fig. 84, are strikingly 



Fig. 83. — Dorsal ver- 

 tebra of Thalattosanrus. 

 (After Merriam.) 



