576 



MAN ATI S AND DUGONGS. 



of those of the hippopotamus. The most interesting points about this animal are 

 the evidences it affords of being a more generalised type than either of its existing 

 allies. Thus the premolar teeth had milk-predecessors, the skull was furnished 

 with distinct nasal bones, and there was a rudimentary hind-limb. 



There is, however, another extinct member of the order, which, although unfor- 

 tunately known only by the skull, presents indications of a still closer affinity with 

 ordinary mammals. This is the Prorastoma, of which the remains have been found 

 in strata, probably belonging to the upper portion of the Eocene period in Jamaica 

 and Italy. This creature had three pairs of incisors, and a pair of canines, as well 

 as seven or eight pairs of cheek-teeth in each, and thus approximated very closely 

 to the ordinary mammalian type ; the front and premolar teeth doubtless having 

 milk-predecessors. Although, therefore, we have not at present actually succeeded 

 in tracing the origin of the Sirenians into terrestrial mammals, yet we have been 

 able to go such a long way in this direction as to leave no doubt that they have 

 been so derived by some evolutionary process. 



