58 GUIDE TO KEPTILES AND AMPHIBIANS. 



of the oval or nearly round shell, Avhich is sculptured externally, and 

 covered with leathery skin instead of horny shields. The toes are 

 extensively connected by webs, but only the three inner ones on 

 each foot are clawed. In the plastron the entoplastral is chevron- 

 shaped. Soft Tortoises are carnivorous, and widely distributed ; 

 they date from the Cretaceous epoch. 



Most of the species belong to the typical genus Trionyx (222-230), 

 nearly allied to which are the Oriental genera Chitra (220) and Pelo- 

 clielys (221), the former distinguished by the elongated skull and 

 forward position of the eyes, and the latter by an intermediate con- 

 dition in these respects. The African Cycloderma (217) and Cyclan- 

 orbis (216), together with the Indian Emyda (219), differ not only 

 in the nature of the sculpture and the form of the bones of the lower 

 shell, or plastron, but likewise in possessing a pair of flaps of skin on 

 the lower surface beneath which the hind-limbs can be withdrawn. 



Many of the species have curious eye-like spots on the back, and 

 the long extensile neck is often marked with yellow spots on a green 

 ground. Indeed, the native Indian name Chitra means spotted. 

 These Tortoises, when of large size, are highly dangerous to bathers. 



Order IX.-SAUROPTERYGIA {extinct). 

 (Case 16.) 



The larger marine Plesiosaurs may be distinguished from the 

 Ichthyopterygia by the absence of a ring of bones in the eye, and 

 by the structure of the paddles, in which the bones, although in 

 excess of the usual number, are more or less elongated, and do not 

 articulate to form a pavement. In the more typical forms the upper 

 arches of the vertebrae are welded to the bodies, with which alone 

 (in all cases) the single-headed ribs articulate. The teeth have 

 separate sockets, and there is but one (the lower) temporal arch. 

 Abdominal ribs are developed on the under surface. The bones of 

 both shoulder-girdle and pelvis develop large ventral plates ; the 

 coracoids and sometimes even the scapulse meeting in the middle 

 line. The skin appears to have been naked. The group ranges 

 from the Trias to the Chalk. 



In the typical Plesiosaurus (336, fig- GO) of the Lias the head is 

 comparatively small and the neck elongated, similar features occurring 

 in the Jurassic Oryptodidns (340) and Mu/re/iosaurns and the Creta- 

 ceous CimoUosaurus, which are distinguished by the structure of the 

 shoulder-girdle and pelvis. In the gigantic Pliosaurus (339) of the 



