144 REPTILES AND BIRDS. 



are placed within the thorax, and articulated to the sides of the 

 vertebral column. 



Of the vertebral column in these extraordinary animals, Pro- 

 fessor Owen remarks that the manifold modifications of the frame- 

 work which render it a portable abode, appear to have been given as 

 a compensation for inferior powers of locomotion, and the absence of 

 offensive weapons. But with all its modifications, the same number 

 of pieces are found in the bony skeleton as in other ordinary ver- 

 tebratae, the form and volume of many of these pieces being alone 

 changed. 



The skin which covers the body of these animals sometimes pre- 

 serves its softness, being altogether devoid of scales; but in nearly 

 all the species it is covered with horny scales of great consistency. 

 Upon the plastron and carapace these scales form large plates, the 

 arrangements and appearances of which vary in different species, 

 some of them being often remarkably beautiful. The material 

 which bears the name of tortoise-shell forms an important article 

 of commerce. 



Aristotle mentions three groups of Tortoises — namely, Land 

 Tortoises, Sea Tortoises, and Fresh-water Tortoises. Cuvier divides 

 them into five sub-genera: — i, Land Tortoises, Testudo ; 2, Fresh- 

 water Tortoises, Emyds ; 3, Marine Tortoises, Chelque; 4, Chelydes, 

 Testudo Jimbriata ; 5, Soft Tortoises, Trionyx — in which he is fol- 

 lowed by Dr. Gray in the British Museum Catalogue, who makes 

 them the third order of Reptiles in his arrangement, as follows : — 



I. Testudinid^e. 

 Testudo, Chersina, Kinixys, Pyxis. 



II. Emydid^e. 



Geoemyda, Emys, Cyclemys, Malaclemys, Cistudo, Kinosternon, 

 Chelydra, Platysternum. 



III. CHELYDIDyE. 



Sternotherus, Pelomedusa, Hydraspis, Chelymys, Phrynops, Chelodina, 

 Hydromedusa, Chelys, Peltocephalus, Padocnemis. 



IV. Trionycid^e. 

 Trionyx, Emyda. 



V. Cheloniad^e. 

 Sphargis, Chelonia, Caretta, Casuana. 



