480 



ZOOLOGY 



SECT. 



formed in the foetus by the separating off of the ventral ends of 

 the ribs. Some of the Cetacea and the Sirenia are exceptional in 

 having a sternum composed of a single piece of bone. The sternal 

 ribs, by which the vertebral ribs are connected with the sternum, 

 .are usually cartilaginous, but frequently undergo calcification in 



old animals, and in some cases become early completely converted 

 into bone. 



The skull of a Mammal (Fig. 1056) contains the same elements 

 and presents the same general regions as that of the Sauropsida, 

 but exhibits certain special modifications. The bones of the skull, 



