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 



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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, 



