64 COPE 



tweeii the Cryptodira and Pleurodira, and they have con- 

 tinued side by side to the present day. The Pleurodira 

 are, however, now confined to the Southern Hemisphere. 

 The Trionychoidea first appear in the Cretacic, and con- 

 tinue as the so-called soft-shell turtles of fresh water. The 

 Athecse first appear in the Eocene, and were then and still 

 are marine in their habits ; but one species now exists, the 

 " leather-back turtle." 



Fiti. 19. — lehUiyosavrus tenuirostis Conyb. ; one. sixth natural size. From the Lias 

 of Wiirtembiirg. From Doederlein. 



The order Ichthyopterygia embraces the families of 

 Ichthyosauridse (Fig. 19) and Mixosauridse. The Mixo- 

 sauridee have the bones of the forearm and leg distinguished 

 by their form from those of the carpus and tarsus, and they 

 thus approach more nearly other reptiles. They are the 

 oldest family, having inhabited European waters during 

 the period of the Trias. In the Ichthyosauridse all the 

 bones of the limbs have the same form, except the humerus 

 and femur. They continued until the close of the Cretacic. 

 They were the especially marine Reptilia, having the 

 shape and habits of the Cetacea. They reached the length 

 of twenty-five feet during the Cretacic period. Their re- 

 mains have been found in all parts of the world except 

 Africa, but they were more numerous in the oceans now 

 covered by Europe than elsewhere. Some of the forms had 

 but few (Mursenosaurus) and others (Baptanodon) no teeth. 

 The typical genera had numerous teeth grooved at the 

 base. The vertebrte are short and biconcave. 



