TORTOISES AND TURTLES. 



25 



which encloses the body, and into which most of the species can 

 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 vertebrae 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 



Fis. 20. 



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



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

 s, shoulder-bones ; j), 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 freshwater 



