26 REPTILE GALLEKY. 



Turtles, digits distinct, armed with sharp claws, and united by a 

 more or less developed membrane or web ; and, finally, the inaiine 

 Turtles, having their limbs transformed into regular paddles, re- 

 sembling those of Cetaceans. The tail is constantly present, but 

 frequently extremely short ; in a few forms only it attains to a con- 

 siderable length. Chelonians are oviparous, and the eggs are 

 generally covered with a hard shell. 



The Chelonians form only a small part of the Class Reptilia, the 

 number of species amounting to about 300. If they occupy in 

 this Gallery almost half of the wall-cases, it is because they are 

 more suited than the other Reptiles for being preserved and exhi- 

 bited in a dried state. 



Chelonians may be divided according to their mode of life into 

 the following Groups : — 



SphargidiB and Cheloniidm, or Sea-Turtles. 

 Trionychidcs, or Treshwater Turtles. 

 Emydid<s and Chelydida, or Freshwater Tortoises. 

 Testudinidce, or Land Tortoises. 



The Sphargidce are a geologically ancient type, in which the 

 formation of a protecting bony carapace has made but little 

 advance. The skin, which in a fresh state is flexible, like thick 

 leather, contains bony deposits arranged like mosaic ; but this 

 dermal shield is not united to the vertebrae and ribs, which remain 

 free, and are not particularly dilated, as may be seen in the large 

 skeleton (G) exhibited opposite to Case 29. In this arrangement 

 the dermal shield and skeleton are in the same relation to each other 

 as in the Crocodiles.. The shape of the limbs is the same as in 

 the marine Turtles, with which the Leather-Turtle agrees in its 

 mode of life; the bones of the paddles, however, have different 

 proportions, they are still more simple, merely rods, and claws are 

 entirely absent. Only one species exists in our time (Dermochelya 

 coriacea), which seems to become gradually rarer, although it is 

 found occasionally throughout all the seas of the tropical and 

 temperate regions, specimens having strayed now and then to the 

 British coast. This Turtle is, perhaps, the largest living Chelonian, 

 exceeding a length of 6 feet, and is said to be herbivorous. 



