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- 
tebrate, 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 :—1, Land Tortoises, Zestudo,; 2, Fresh- 
water Tortoises, Emyds ; 3, Marine Tortoises, Che/gue; 4, Chelydes, 
Lestudo fimbriata ; 5, Soft Tortoises, 77¢éonyx—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. TESTUDINIDZ. 
Testudo, Chersina, Kinixys, Pyxis. 
II. EMyDIDA. 
Geoemyda, Emys, Cyclemys, Malaclemys, Cistudo, Kinosternon, 
Chelydra, Platysternum. 
III. CHELYDIDA. 
Sternotherus, Pelomedusa, Hydraspis, Chelymys, Phrynops, Chelodina, 
Hydromedusa, Chelys, Peltocephalus, Padocnemis. 
IV. TRIONYCID. 
Trionyx, Emyda. 
V. CHELONIADA. 
Sphargis, Chelonia, Caretta, Casuana. 
