CHELONIA. 401 



known Turtles (Cheloniidce), all of which swim with great ease 

 and power, but are comparatively helpless upon the land (fig. 

 156). The best-known species are the "edible" or Green 

 Turtle (Chelonia mydas), the Loggerhead Turtle (Chelonia caou- 

 anna), the Hawk's-bill Turtle (C. imbricata), and the Leathery 

 Turtle (Sphargis coriacea). The Green Turtle is largely im- 

 ported into this country as a delicacy, and occurs abundantly 

 in various parts of the Atlantic and Indian Oceans. The 

 Hawk's-bill Turtle is of even greater commercial importance, 

 as the horny epidermic plates of the carapace constitute the 

 " tortoise-shell" so largely used for ornamental purposes. The 

 Leathery Turtle is remarkable in having the carapace covered 

 with a leathery skin in place of the horny plates which are 

 found in other species. 



In the second section of the Chelonia, in which the limbs are 

 adapted for an amphibious life, are the Mud-turtles or Soft 

 Tortoises (Trionycida), and the Terrapenes (Emydida). In 

 the Trionycidcz the development of the carapace is imperfect, 

 the ribs being expanded and united to one another only near 

 their bases, and leaving apertures near their extremities. The 

 entire carapace is covered by a smooth leathery skin, and the 

 horny jaws are furnished with fleshy lips. All the Trionytida 

 inhabit fresh water and are carnivorous in their habits. One 

 of the largest and best known is the so-called Snapping Turtle 

 (Trionyx ferox) of North America, but other species are found 

 in Egypt and in the East Indies. The Terrapenes (Emys) 

 have a horny beak, and have the shield covered with epider- 

 mic plates. They are inhabitants of fresh water, and are most 

 of them natives of America. 



The third section of the Chelonia comprises only the Land 

 Tortoises (Testudinidce), in which the limbs are adapted for ter- 

 restrial progression, and the feet are furnished with short nails. 

 The carapace is strongly convex, and is covered by horny epi- 

 dermic plates ; the head, limbs, and tail can be completely re- 

 tracted within the carapace. Though capable of swimming, 

 the Tortoises are really terrestrial animals, and are strictly 

 vegetable-feeders. The most familiar species is the Testudo 

 Graca, which is indigenous in Spain, Italy, and Greece, but is 

 commonly kept in this country as a domestic pet. 



DISTRIBUTION OF CHELONIA IN TIME. The earliest known 

 traces of Chelonians occur in the Permian Rocks, in the lower 

 portion, that is, of the New Red Sandstone of older geologists. 

 These traces, however, are not wholly satisfactory, since they con- 

 sist solely of the footprints of the animal upon the ripple-marked 

 surfaces of the sandstone. Of this nature is the Chelichnus 



VOL, II. 2 C 



