THE SIRENIA, OR SEA-COWS. 245 



as Coryphodon, that still showed the old five-toed 

 extremity, an inheritance from Pre-Tertiary times. 

 However all the living Sirenians possess a five- 

 fingered hand. When, therefore, it is said that the 

 molars of Prorastomus are genuine ridged teeth, 

 these do not point to the true Lophiodonta and 

 tapirs, with their already reduced hand, but to 

 earlier ancestors on both sides. Thus things no 

 longer existing point to that very distant past, 

 which extends back beyond our actual observa- 

 tions. Even in the case of Halitherium all that 

 is left of the hind limb is the thigh-bone. This 

 bone, however, is still attached to the pelvis, 

 which is tolerably reduced, but has a socket. The 

 earliest Sirenians, therefore, had a less striking 

 form of skull, but, nevertheless, in their whole 

 appearance were already like the present living 

 species. From this it follows that the four- 

 footed mammals changed their abode for the sea, 

 and lost their hind limbs, before the Tertiary 

 period. 



The living Sirenians have experienced a still 

 farther reduction of the pelvis ; it has become de- 

 tached from the vertebral column ; and even the 

 above-mentioned remnant of the hind limb, the 

 rudimentary thigh-bone, has wholly disappeared. 



