2i6 CHARACTERS OF VERTEBRATE ANIMALS 



the crocodile, but the internal nostrils open on the roof of the 

 mouth, as the habits of the animal do not require any special 

 provision to be made for a distinct breathing tract as distinct 

 from the cavity of the mouth. It may also be remarked that 

 the immobility of the trunk presents a certain obstacle to the 

 rapid renewal of the air in the lungs, and this is probably the chief 

 reason for the sluggishness of chelonians. 



The brain presents the usual parts, and is, for a reptile, of 

 comparatively high type, being only inferior to that of the croco- 

 dile. 



Tortoises are developed from hard-shelled eggs much like 

 those of a bird. About a dozen of these are deposited together 

 in a hole scooped out in the earth, and are afterwards carefully 

 covered up. A warm spot is selected for the purpose, and the 

 eggs, which are laid in early summer, are hatched out by the heat 

 of the sun. 



The following groups are recognized among the Chelonia, 

 and in dealing with them space will only permit of allusion to 

 some of the more interesting forms: — 



(i) Leathery Turtles. 



(2) S-necked Chelonia. 



(a) Land and Fresh-water Tortoises. 

 \b) Turtles. 



(3) Side-necked Tortoises. 



(4) Soft Tortoises. 



(i) Leathery Turtles. — This group contains but a single 

 species, the Leathery Turtle or Luth (Sphargis or Dermatochelys 

 coriacea) (fig. 135), which differs in many important particulars 

 from other Chelonia, not only from the terrestrial and fresh-water 

 forms, but also from the Turtles proper, which, like it, are marine. 

 It is the only living representative of an important extinct group, 

 some members of which attained a very considerable size, and 

 is the largest existing marine species, being when full-grown 

 over 6 feet long. The limbs are modified into large nailless 

 flippers, which are very efficient swimming organs. These, how- 

 ever, are not its special peculiarity, for similarly modified limbs 

 are found in ordinary turtles, but this is found in the nature of 

 the " shell ". The carapace is made up of numerous bony pieces 

 united together in a tesselated manner and quite free from the 



