TOETOISES. 157 



lateral margins of the spinous processes of the vertebras, so that 

 they form together a single broad plate ; the heads of the ribs are 

 feebly developed, and the intervals between them and the bodies 

 of the vertebrae filled up with ligament. The margin of the shield 

 thus formed by the dorsal ribs is further enlarged by a third set 

 of flat bones fixed by sutures around the whole circumference of 

 the carapace. 



The plastron, or ventral plate, is made up of nine pieces, of 

 which eight are arranged in pairs ; but the ninth, which is always 

 placed between the four pieces composing the two anterior pairs, 

 is single, and occupies the mesial line. The bones of the shoulder 

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

 of the vertebral column. 



Of this vertebral column in these extraordinary animals, Pro- 

 fessor Owen remarks that the manifold modifications of the 

 framework 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 vertebratse, the form and volume of many of these pieces 

 being alone changed. 



The skin which covers the body of these animals sometimes 

 preserves its softness, being altogether devoid of scales ; but in 

 nearly all the species it is covered with horny scales of great con- 

 sistency. Upon the plastron and carapace these scales form large 

 plates, the arrangements and appearances of which vary in dif- 

 ferent 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, 

 Xepo-ata, or Land Tortoises ; OaXarrla, or Sea Tortoises ; and 

 or Freshwater Tortoises. Cuvier divides them into five sub- 

 genera : 1, Land Tortoises, Testudo (Brogniart) ; 2, Fresh- water 

 Tortoises, Emydes (Brogniart) ; 3, Marine Tortoises, Chelque 

 (Cuvier) ; 4, Chelydes, Testudo fimbriata ; 5, Soft Tortoises, Trionyx 

 (Geoffrey) in which he is followed by Dr. Gray in the British 



