576 
MAN A TIS ANN 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. 
