2460 



TORTOISES, TURTLES, AND PLESIOSAURS 



Both groups have the knver bones of the pelvis expanded into large flat plates, and 

 there is also a similarity in the structure of the bones of the limbs. 



Whereas, however, the tortoises have the upper surface of the body covered 

 with a shell, and the lower aspect of the same protected by a plastron, the 

 plesiosaurs were entirely naked, the plastron being represented by a numerous 



RESTORED SKELETON OF A PLESIOSAUR. 

 (Greatly reduced.) 



series of abdominal ribs, each composed of three pieces, forming a forwardly- 

 directed angle. The skull differs from that of the crocodiles in having but one 

 (lower) temporal arch; and the jaws are furnished with a number of pointed and 

 grooved teeth, implanted in distinct sockets; one of such teeth being figured 

 on p. 2361. The neck was generally much elongated, and its vertebrae differ from 

 those of crocodiles in that their ribs which may have either single or double 



heads, are articulated only to the body of 

 each vertebra (as shown in the accom- 

 panying figure); those of crocodiles al- 

 ways having two heads, of which the 

 lower is articulated to the body, and 

 the upper to the arch of the vertebra. 

 Throughout the backbone the bodies of 

 the vertebrae have either nearly flat or 

 slightly-cupped articular surfaces; and 

 in the region of the back each pair of 

 ribs is articulated to a process arising 

 from the arch of each vertebra, instead 



UPPER SURFACE OF THE SKULL OF THE 

 SIMOSAUR OR TRIASSIC PLESIOSAUR. 



(One-fourth natural size. ) 



n. nostrils; or. eye socket; st. temporal fossae. 

 (From Gaudry.) 



of from a facet placed at the junction of 

 two vertebrae, as in the tortoises. Al- 

 though there are other interesting fea- 

 tures in these reptiles, those mentioned distinguish them from crocodiles and 

 dinosaurs on the one hand, and tortoises and turtles on the other. 



With regard to the various groups into which the order is divided, it may 

 be mentioned that in the typical forms, constituting the family Plesiosauridce, the 

 limbs, as shown in the above figure, are converted into flattened paddles, with 



