554 



REPTILIA. 



around the whole circumference of the Carapax, which they assist 

 in completing. p . g ^ 



(608.) The Plastrum, or Sternum, is made up of nine pieces, 

 which have been proved by M. Geoffroy St. Hilaire to be the ele- 

 ments of this portion of the skeleton in the most complete state of 

 developement in which they are met with. Of these nine elements, 

 eight are disposed in pairs ; but the ninth, which is always placed 

 between the four pieces composing the two anterior pairs, is single, 

 and occupies the mesial line: in birds we shall afterwards find 

 this element of the sternum performing a very important office. 



(609.) The bones of the shoulder, and of the hip, in the Tor- 

 toise (fig- ^50), are absolutely placed within the thorax, and articu- 

 lated to the sides of the vertebral column. The precise homology 



