n6 4 MANATEES AND DUGONGS 



in some respects a kind of connecting link between the manatee and the dugong. It 

 resembled the latter in having the extremities of the jaws deflected, and in the 

 presence of a pair of tusks in the upper jaw; but its molar teeth were more like 

 those of the manatee, although with a pattern recalling that obtaining on the 

 crowns of those of the hippopotamus. The most interesting points about this ani- 

 mal are the evidences it affords of being a more generalized 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 un- 

 fortunately 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 jaw, 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. 



