TORTOISES AND TURTLES. 25 



retract their head and limbs. This armour consists of two shields 

 united by their lateral margins ; the upper, or carapace, is formed 

 by the expansion and union of the vertebrse and ribs ; the lower, 

 or plastron, by dermal bones only. In most of these animals the 

 carapace presents three series of central bony plates — the vertebral 

 medially, and the costal laterally — and they are surrounded by a series 



Fig. 20. 



Skeleton of Tortoise, in a vertical section through the carapace. 



c, neck ; v, dorsal vertebrse ; t, tail ; r, costal plates ; pi, plastron ; 

 s, shoulder-bones ; p, pelvis. 



of marginal plates ; the plastron bones are generally nine in number, 

 one median and four pairs. Horny epidermic plates cover the 

 carapace and plastron ; their arrangement is also symmetrical, 

 but by no means corresponds to that of the underlying bones; 

 they constitute what is called the " Tortoise-shell," which in some 

 species has great commercial value. The jaws are toothless, 

 covered by a horny bill, rarely hidden under fleshy lips. The 

 four limbs are always well developed, and modified according to 

 the mode of life of the species, — the terrestrial Tortoises having 

 short, club-shaped feet furnished with blunt claws; the ireshwater 

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

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



