158 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



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 

 hawkbill turtle, which furnish the "tortoise shell" of 

 commerce, have the limbs converted into paddles. The 

 fresh-water forms, represented by the snapping turtle 

 (Chelydra), 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. 



3. Crocodilia, the highest and largest of reptiles, have 

 also two exoskeletons one of horny scales (epidermal), 



FIG. 138. Alligator (A. misszssz^zensz's) . Southern States. 



and another of bony plates (dermal). The bones of the 

 skull are firmly united, and furnished with numerous 

 teeth, implanted in distinct sockets. The lower jaw 

 extends back of the cranium. The heart has four 

 cavities, but the pulmonary artery and aorta commu- 

 nicate 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 skin, and eyes with 

 movable lids ; a muscular gizzard ; a long, compressed 



