io8 ANIMAL LIFE PAST AND PRESENT. 



probable that the bony plastron of the tortoises is an 

 extreme development of the peculiar system of so- 

 called abdominal ribs found on the lower surface of 

 the body in crocodiles and many extinct groups of 

 reptiles, such as the Fish-Lizards and Plesiosaurs ; and 

 it appears that the nearest allies of the Chelonians are 

 to be found in the latter group. 



Chelonians vary greatly in their habits, and the 

 nature of their food. Thus the land- tortoises, as their 

 name implies, are terrestrial, although all of them can 

 swim ; while some closely allied types are aquatic. 

 The feet of the former are provided with strong claws, 

 and their food consists exclusively of vegetables, of 

 whicli they consume a vast amount. The pond-tortoise 

 of Southern Europe, which, with the exception of the 

 common Grecian land-tortoise, is the only European 

 representative of the group, has, however, the feet 

 webbed, and subsists on animal substances, such as 

 worms. Many of the Indian water-tortoises are also 

 carnivorous; this being especially the case with the 

 well-known soft-tortoises of the rivers of the warmer 

 parts of the globe. In those reptiles the shell is not 

 provided with horny shields, but has the outer surface 

 of the bones ornamented with a beautiful net -like 

 sculpture, and merely covered with a thin skin. In 

 the turtles, again, which are of marine habits, although 

 they resort to the shore to deposit their eggs, the limbs 

 have all the toes enclosed in a common scaly integu- 

 ment, so as to form paddles admirably adapted for 

 swimming organs. While, however, the edible Green- 



