CIIELONIA 223 



In recent years, however, the earlier members of the older group 

 of Pleurodira have been separated into a fourth suborder, the 

 Amphichelydia, a group characterized by some not very impor- 

 tant differences in the plastron and skull, and including those 

 forms in which the cervical vertebrae are amphicoelous. This 

 group continued to Eocene times before it became extinct, and 

 consisted of archaic forms which persisted after all the other sub- 

 orders had come into existence. The Cryptodira, especially char- 

 acterized by the manner in which they withdraw the head and neck 

 within the shell by an S-like vertical flexure, are known from the 

 Lower Jurassic and are still the dominant group of today, with more 

 than one hundred and forty living species. The Pleurodira in the 

 narrower sense are first known from their remains in the Upper 

 Cretaceous of North America and are still represented by about 

 forty species, living in the Southern Hemisphere. They are dis- 

 tinguished from the other groups by the manner in which they with- 

 draw the neck and head into the shell, by a horizontal, sidewise 

 flexure. The third suborder, the Trionychoidea, also began in 

 Cretaceous times, so far as we know, and are represented by about 

 seventy living species, chiefly in the Northern Hemisphere. They 

 are especially characterized by the absence of bony marginal 

 plates and the soft epidermis. 



With the exception of the land tortoises, all turtles from the 

 beginning of their career as an order to the present time have been 

 more or less at home in the water. In some, like the marine forms, 

 the adaptation to aquatic life has produced marked changes in 

 structure: in the loss of the horny dermal shields and in the loss 

 of bone tissue; in the flattening of the shell, and in the development 

 of the front legs into swimming flippers, with a loss of the claws. 

 In the absence of a guiding tail, which is always small in the marine 

 turtles, propulsion must of course be wholly by the aid of the limbs. 

 As oar propellers the marine turtles show some of the peculiar 

 characters of the plesiosaurs. With a like short and broad body, 

 a more or less elongated and flexible neck, there could be no sinuos- 

 ity of the body in swimming. As an oar-like organ the humerus 

 became flattened, and its muscular attachments, as in the plesio- 

 saurs, descended far down the shaft, giving greater mechanical 



