VERTEBRATA. 323 



bones, united to the expanded spines of the vertebrae and 

 to the ribs, making the walls of the carapace. The ven- 

 tral pieces form the 

 plastron, or ster- 

 num. 1M All are 

 toothless. There 

 are always four stout 

 legs; and the order 

 furnishes the only 



examples Of Vei'te- F IQ . 302. Box-tortoise (Cistudo Virginea). United 



brates lower than states. 



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 

 (Chelydro), 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 



