CHELONIA 225 



pelvis with the plastron below, which never occurs in other turtles, 

 unless it be the Amphichelydia. 



The side-necked turtles are all of fresh-water habit, similar to 

 that of the fresh-water tortoises spoken of farther on. The neck 

 is often very long and snake-like, which accounts for one of the 

 names given to these turtles; because it is withdrawn into the 

 shell sidewise, it has more distinctively ball-and-socket joints 

 between the vertebrae, with distinct transverse processes for the 

 attachment of the necessary side-moving muscles. The feet in all 

 are more or less webbed and armed with strong claws. 



The largest and perhaps the best known of all living side- 

 necked turtles is the giant Amazon turtle of South America, which 

 sometimes has a shell nearly three feet in length. Its feet are 

 broadly webbed, and the shell is rather flat in the adult; it is an 

 excellent swimmer in the waters of the Orinoco and Amazon. Six 

 or seven species of the genus to which it belongs are known, all 

 of them South American except one that lives in Madagascar 

 and one i^ossil found in the Eocene of India. This remarkable 

 distribution is but one more of the many instances known in 

 zoology and paleontology that seem to prove an early land con- 

 nection between India and South America. Had the migration 

 between the two continents occurred by way of Asia and Bering 

 straits, as did that of hosts of mammals, one would certainly expect 

 to find some evidence of it in the North American Tertiary rocks, 

 which, so far, is lacking. 



CRYPTODIRA 



The chief families of the Cryptodira turtles are the Chelydridae, 

 or snappers; the Emydidae, or marsh tortoises; the Testudinidae, 

 or land tortoises; the Chelonidae, or sea-turtles; the Protostegidae, 

 or ancient sea-turtles; and the Dermochelydidae, or leather-backs. 

 Other doubtful or smaller groups, both living and extinct, may be 

 omitted, or incidentally mentioned. 



SNAPPING TURTLES 



The family of snapping turtles, the Chelydridae, are of interest 

 because of their peculiar geographical distribution at the present 



