THE SPINAL COLUMN OF REPTILES. 155 



Few persons who look at a tortoise consider 

 that the shell, as it is termed, into which the 

 animal can withdraw its limbs and head, con- 

 sists, in reality, of a great portion of the 

 skeleton developed externally, and modified 

 into a box or case in which the muscles, the 

 bones of the shoulders, and hips, and the 

 viscera are inclosed ; this box is covered with 

 horny plates, sometimes with a leathery mem- 

 brane. The arched upper portion of this case 

 is termed the carapace, the lower is called the 

 plastron. Now let us see what the carapace 

 really is, and of what it is composed. If we 

 remove the plastron, we at once perceive the 

 dorsal portion of the spinal column running 

 down the centre of the concave inside of the 

 carapace, and consolidated to it so as to be 

 immovable ; the individual vertebrae, however, 

 may be readily distinguished — they are small 

 and elongated. On looking more narrowly, and 

 supposing the horny plates on the outside to be 

 removed, we find a series of bones, imited 

 together by suture like those of the human 

 skull, running down the central portion of 

 the carapace — these bones are the processes 

 of the vertebrse thus strangely altered from 

 their ordinary form and appearance. On each 



