CHELOMAN REPTILES. 



41. 'J 



parts is really formed by an extension 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 sterno-costal appendices, which 

 become ossified. Thus no new element has been 

 created ; but advantage has been taken of those 



already existing in the general type of the verte- 

 brata, to modify their forms, by giving them dif- 

 ferent degrees of relative developement, and con- 

 verting them, by these transformations, into a me- 

 chanism of a very different kind, and subservient 

 to other objects than those to which they are 

 usually applied. 



The first step taken to secure the relative immo- 



