208 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



tion of their organs to their aquatic environment. 

 All palaeontological data relative to this group, which 

 habitually dwells on the seashores and in the mouths 

 of great rivers (Siren and Lamantin [Manati]), favour 

 the hypothesis of Owen and Flower, who consider 

 these beings as early terrestrial Ungulates, which 

 have become aquatic and whose bodily form, skin, 

 dental system, skull, and especially limbs, have 

 undergone modifications in harmony with their 

 new environment. The most primitive and no 

 doubt the earliest genus, the Prorastomus of the 

 Jamaica Eocene, is distinguished from all other 

 Sirenians by the almost normal dentition of an 

 Ungulate, with incisors and canines in function in 

 both jaws, by molars with transverse ridges, by 

 small maxillaries not directed downwards, as in the 

 Haliiherium of the Oligocene, whose incisors become 

 at once rudimentary and useless, and in which the 

 canine transform themselves into a pair of strong 

 tusks. A still more important functional modifica- 

 tion happens to the limbs : the anterior limb, an 

 essential organ for propulsion through the water, 

 retains its normal structure, except that the meta- 

 carpal bones and the phalanges lengthen so as to 

 support a powerful paddle. On the other hand, 

 the posterior limb, useless to a lengthened and 

 pisciform body, is gradually atrophied. In the 

 Haliiherium of the Oligocene and Miocene a small 

 basin hollowed out of a small cotyloiid cavity exists 

 and is furnished with a thin femur in the shape of a 

 rod : in the Metaxytherium of the Pliocene, this 

 basin persists and still shows a certain width at the 

 level of the ilium, but there is no longer any cotyloid 



