VERTEBRATA. 
323 
Fig. 302. — Box-tortoise (Cistudo Virginea). United 
States. 
bones, united to the expanded spines of the vertebrse and 
to the ribs, making the walls of the carapace. The ven¬ 
tral pieces form the 
plastron , or ster¬ 
num. 166 All are 
toothless. There 
are always four stout 
legs; and the order 
furnishes the only 
examples of Verte¬ 
brates lower than 
Birds that really walk, for Lizards and Crocodiles wrig¬ 
gle, and drag the body along. There are no teeth, but a 
horny beak. The eggs are covered with a calcareous 
shell’. 
The Sea-turtles, as the edible Green Turtle and the 
Hawk’s-bill Turtle, which furnish the “tortoise-shell” 
of commerce, have the limbs converted into paddles. The 
fresh-water forms, represented by the Snapping Turtle 
( Ghelydra ), are amphibious, and have palmated feet. Land 
Tortoises ( Testudo ) have short, clumsy limbs, fitted for 
slow motion on the land; the plastron is very broad, and 
the carapace is arched (while it is flattened in the aquatic 
species), and head, legs, and tail can be drawn within it. 
The land and marine species are vegetable-feeders; the 
others, carnivorous. 
4. Crocodilia , the highest and largest of Reptiles, have 
also two exoskeletons—one of horny scales (epidermal), and 
another of bony plates (dermal). The bones of the skull 
are firmly united, and furnished with numerous teeth, im¬ 
planted in distinct sockets. The lower jaw extends back 
of the cranium. The heart has four cavities, but the pul¬ 
monary artery and aorta communicate with each other, so 
that there is a mixture of venous and arterial blood. 
They have external ear-openings, closed by a flap of the 
