218 



BACKBONED ANIMALS, 



The Double - Walkers (AmphisbcRnidx) (Fig. 262) 

 are found in tropical America, often in the nests of ants. 

 They move in either direction with equal ease. Thirteen 

 species are known in Asia, Africa, and South America. 



FIG. 262. Amphisbanafuliginosa. 



Order III. Turtles (Chelonia). General Characteris- 

 tics). The turtles (Fig. 263) are distinguished by the shell 

 or box-like covering that envelops them. The upper part 

 forms the carapace, r, the lower the plastron, //, the two 

 constituting a covering into which the head, tail, and limbs 

 can be more or less withdrawn. The generally arched 

 carapace is formed by the greatly expanding ribs, these 

 and the vertebrae being fixed and immovable. The plas- 

 tron is generally considered a greatly expanded sternum.* 

 The outer surface of the shell is made up of scales or 

 plates, or a leathery substance, as in the case of the soft- 

 shell tortoise. The jaws are toothless, being armed with a 

 horny beak, as in the birds. The eyes have three lids ; 



* Some naturalists consider it a dermic growth. 



