GENERAL CHARACTERS OF REPTILES. 525 



animal can generally withdraw itself completely. One of these 

 shields occupies the back, and is named the carapace ; the other, 

 situated under the body, is called the plastron. (Fig. 327.) 

 They are united together on each side, in such a manner as to 

 leave, in front and behind, an opening that serves for the pas- 

 sage of the head, feet, and tail. This kind of cuirass is only 

 covered by the skin, which, in its turn, is generally furnished 

 with large scaly plates ; and all the muscles and other soft parts 

 are enclosed in the great cavity thus formed. 



464. The osseous framework of Tortoises, in order to present 

 this unusual arrangement, must necessarily, it is evident, be 

 greatly modified : we nevertheless find the same constituent parts 

 as in ordinary Yertebrata ; but several of these pieces have 

 changed their form and size. When we examine the carapace on 

 its upper surface, we see that it is formed by a great number of 

 bony plates, united together by sutures ; of these plates, eight 

 occupy the median line, sixteen constitute a longitudinal range on 

 each side of these, and twenty-five or twenty-six surround the 

 whole like an oval frame. It is difficult then to recognise the 

 nature of these bones ; but if we examine the carapace by its 

 lower surface (Fig. 305), we immediately see that the central 

 pieces, of which we have just spoken, are nothing else than appen- 

 dages of the dorsal vertebrae. On the under side, the body of each* 

 of these bones is found, in fact, to present its ordinary form ; as 

 is also the vertebral canal, which serves to lodge the spinal cord ; 

 but the upper portion of the walls of the ring which constitute 

 this canal, instead of having its usual form of a simple trans- 

 verse arch of bone separated by a void space from those above 

 and below, and instead of being surmounted by a spinous process, 

 is here spread out sideways as a disk, and is continuous without 

 interruption with the corresponding plates, belonging to the ver- 

 tebra which precedes, and to that which follows. These dorsal 

 vertebrae, thus become immovable, have attached to each a pair 

 of ribs, as in Man and most other Yertebrata ; but these ribs (c) 

 are so much widened, as to touch each other along the whole, or 

 nearly the whole, of their length, and are connected together by 

 sutures. Finally, the marginal pieces, which are articulated with 



