REPTILES 1395 



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 structures. The great pur- 

 pose 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 

 pressures from without, and proof against any ordi- 

 nary measures 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 can not fail to 

 strike us in viewing the dispositions of the parts of 

 the skeleton and the remarkable inversion they ap- 

 pear to have undergone, 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 ex- 

 tension of the spinous processes of the vertebrae and 

 ribs on the one side and of the usual pieces which 

 compose the sternum on the other. The upper and 

 lower plates thus formed are united at their edges 

 by expansions of the sternocostal appendices, which 

 become ossified. Thus, no new element has been 

 created; but advantage has been taken of those al- 

 ready existing in the general type of the vertebrata, 

 to modify their forms by giving them different de- 



