PLESIOSAURS. 



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 

 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. 5. 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 



restored skeleton of a plesiosaur (greatly reduced). 



heads, are articulated only to the body of each vertebra (as shown in the accom- 

 panying figure); those of crocodiles always 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 vertebras 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 of 

 from a facet placed at the junction of two vertebrae, as in the tortoises. Although 

 there are other interesting features 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 figure on p. 102, 

 are converted into flattened paddles, with 

 a shortening of the bones of the upper 

 segments, and an increase in the number 

 of bones corresponding with those of the 

 toes of ordinary reptiles. In the true 

 plesiosaurs (Plesiosaiiriis) of the Lias, the 

 ribs of the neck were articulated to the vertebrae by two heads : whereas in the 

 later cimoliosaurs (Cimoliosaurus) of the Oolites and Chalk, such ribs, as shown in 

 the figure of a neck -vertebra, were single-headed. Some of these creatures were 

 of huge size, attaining a length of between 30 and 40 feet ; certain of the species 



upper surface of the skull of the simosaur, 

 or triassic plesiosaur. 



n, nostrils ; or, eye-sockets ; st, temporal fossa; 

 (| nat. size). — From Gaiulry. 



