2446 



TORTOISES, TURTLES, AND PLESIOSAURS 



Gigantic as is the existing leathery turtle, it was considerably exceeded 

 by some of its extinct allies. Among these, the huge Eosphargis from the 

 London Clay, with a skull of nearly a foot in length, apparently had a cara- 

 pace consisting only of one median row of broad-keeled bony plates, and a 

 border of marginal bones; while in Psephophoms from the higher Eocene and 

 Miocene strata of the Continent, both upper and lower shells were formed of 

 mosaic-like bones, which, it is thought, were overlain by horny shields. In the 

 earlier Protostega and Protosphargis from the Cretaceous rocks of North America 

 and Europe, the upper shell appears to have been represented merely by a row of 

 marginal bones, while the lower one was very stoutly ossified; some of these early 

 forms probably attained a length of from ten to twelve feet. 



LOWER AND SIDE VIEWS OF SKULL AND UPPER AND LOWER ASPECTS OF 



LOWER JAW OF GREAVED TORTOISE. 



(From Gray, Proc. Zool. Sac., 1870.) 



THE SIDE-NECKED TORTOISES 

 Families ClIELTID^ and PELOMEDUSID^ 



In place of withdrawing the head into the shell by means of an S-like flexure- 

 of the neck in a vertical plane, as in all the groups hitherto described, the re- 

 mainder of the living tortoises with complete shells bend the neck sideways in a 

 horizontal plane (as shown in the illustration on page 2450), and thus bring the 

 head within the margins of the shell. Accordingly, the group is collectively 



