218 BACKBONED ANIMALS. 
The Double- Walkers (Amphisbenida) (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. 
Fic. 262.—Amphisbena fuliginosa. 
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, 7, the lower the plastron, f/, 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can be more or less withdrawn.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 vertebre 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. 
