Not so the River-tortoises,[77] which are greedy animal-feeders, and as they live in the water do not need the same protection. Their box is much flatter and more open at the ends, so as to allow them to swim freely with their webbed feet; and they are fierce and bold, the Snapping Turtle[78] of the lakes and rivers of America being a terrible fellow, tearing the frogs and fishes in the water with his sharp claws, and even snapping strong sticks in half with his powerful beak. The Mud-tortoises, too, which swim swiftly with their strong legs and long neck outstretched, do not need a hard shell, and they have scarcely any plate below, and only a gristly leathery covering above, which looks very like the mud in which they hide. Lastly the Sea-tortoises or Turtles, which swim in the warm parts of the Atlantic and Pacific Oceans, have only an open flat shell under which they cannot draw their head and feet, for they strike out boldly into the open ocean, feeding on seaweed, jelly-fish, and cuttle-fish, rowing grandly along with their broad paddles which they feather like oars as they go. They have only one time of weakness--when they come on islands, such as Ascension and the Bahama Islands, which they choose probably because they find fewer large animals there. There the mother turtle arrives at night, looking fearfully around, and if all is still comes flapping in over the sand, and, clearing a hole with her flippers, lays about 200 soft round eggs and covers them up and leaves them. Then in about a month the young turtles come out and make at once for sea, though many of them fall victims to large birds of prey on their way. Woe, too, to the mother when she is laying her eggs, if these large birds are near, for she cannot defend her soft body; or, worse still, if the natives are on the look-out; for then the Green Turtle,[79] coming ashore from the Atlantic, is tilted over on her back and killed for food; and the Hawk’s-bill Turtle[80] from the Indian or Pacific Oceans is cruelly stripped of its shell for ornaments. Yet they must run these risks, for their eggs would not hatch without the warm sun, and we see how great is the gap between the last water-breathers and the first air-breathers, when we remember that the frogs go back to lay their eggs in the water, while the tortoises, even when they live far out at sea, are forced to come in to shore, in spite of great dangers, to lay their eggs that their little ones may begin life upon land. * * * * * [Illustration: Fig. 23. Skeleton of a Lizard. _sp_, Spinous processes, which in the tortoise are flattened into plates; _r_, ribs; _s_, shoulder bone; _a_, upper arm; _e_, elbow; _fa_, forearm; _h_, hip bone; _th_, thigh bone; _k_, knee; _l_, bones of the leg; _q_, quadrate bone between upper and lower jaw. ]