236 VERTEBRATE ZOOLOGY 



DIVISION 1. CRYPTODIRA 



In these turtles the carapace is covered with horny scutes: the 

 neck is retractile, bending chiefly in a vertical plane; and the pelvis 

 is not fused with the shell. By far the majority of our common 

 turtles and tortoises belong to this division. 



Family 1. Chelydridae (Snapping Turtles) The common snapper 

 (Chelydra serpentina) and the alligator snapper (Macrochelys 

 temmincki), both North American species, are the only living rep- 

 resentatives of this primitive family. The common snapper 

 (Fig. 133, C) is our most generalized modern turtle. Its head, 

 body and tail are rather evenly balanced, and the limbs are propor- 

 tionally heavy and typically reptilian. There is also less complete 

 boxing in of the movable parts than in most other species. In the 

 tail of Chelydra are found not only the rows of plates and scutes that 

 are homologous with those in the armature, but at least five rows that 

 have disappeared from or are merely vestigial in the latter. Hence 

 the ancestral condition of the armature is probably more nearly du- 

 plicated in the basal portion of the tail of Chelydra than anywhere else. 

 The snapper is a slow and clumsy creature, exceedingly sullen and ill- 

 tempered in captivity. When irritated it snaps blindly with widely 

 open mouth, and seizes indiscriminately any object within reach. It is 

 decidedly aquatic in habit and is not fond of basking in the open. 

 More of ten it is found in shallow, warm pools partly buried in the mud. 

 At times it goes on journeys cross-country from one body of water to 

 another. The snapper makes its nest in loose gravelly or sandy soil at 

 no great distance from the water's edge, though it may wander some 

 distance inland before selecting a suitable nesting place. In excavating 

 the nest a shallow, funnel-like depression is first made; then a crude 

 tunnel is scraped out and enlarged at the bottom into a chamber. 

 All of the digging is done with the hind feet, which are armed with 

 heavy claws. About thirty to forty spherical eggs with tough elastic 

 shells are laid layer on layer with pads of sand packed between; and 

 a layer of sand is packed in and smoothed over the top. Chelydra 

 is carnivorous, feeding on fish, frogs, young ducks and all other 

 aquatic animals that come its way. Active prey is caught by 

 stealth. The dull, mud-colored body renders it inconspicuous and 

 aids it in slipping up close to an unwary frog or fish. If the snapper 

 ever approaches a prospective victim so as to be able to snap its 



