CHELONIAN REPTILES. 321 



§ 5. Chelonia, 



The order of Chelonian Peptiles, which comprises all 

 the tribes of Tortoises and Turtles, appears to constitute an 

 exception to the general laws of conformation, which pre- 

 vail among Vertebrated Animals: for instead of presenting 

 a skeleton wholly internal, the trunk of the body is found 

 to be enclosed on every side in a bony case, which leaves 

 openings only for the head, the tail, and the fore and hind 

 extremities. That portion of this osseous expansion which 

 covers the back is termed the Carapace; and the flat plate 

 which defends the lower part of the body is termed the 

 plastron. It is a form of structure that reminds us of the 

 defence provided for animals ver}'' low in the scale of or- 

 ganization, such as the echinus, the Crustacea, and the bi- 

 valve mollusca. Yet the substance which forms these strong 

 bucklers, both above and below, is a real osseous structure, 

 developed in the same manner as other bones, subject to all 

 the changes, and having all the properties of these struc- 

 tures. The great purpose which nature seems to have had 

 in view in the formation of the Chelonia is security; and for 

 the attainment of this object she has constructed a vaulted 

 and impenetrable roof, capable of resisting enormous pres- 

 sures from without, and proof against any ordinary mea- 

 sures of assault. It is to the animal a strong castle, into 

 which he can retire on the least alarm, and defy the efforts 

 of his enemies to dislodge or annoy him. 



These considerations supply us with a key to many of 

 those apparent anomalies, which cannot fail to strike us in 

 viewing the dispositions of the parts of the skeleton (Fig. 

 213,) and the remarkable inversion they appear to have un- 

 dergone, when compared with the usual arrangement. We 

 find, however, on a more attentive examination, that all the 

 bones composing the skeleton in other vertebrated animals 

 exist also in the tortoise; and that the bony case which en- 

 velops all the other parts is really formed by an extension 



Vol. I. 41 



