MAEINE MAMMALS 279 



manatee (Trichechus manatus), as it is called, does not in- 

 habit the open ocean. It frequents shallow bays and lagoons, 

 where it browses peaceably on seaweeds, just as cattle graze 

 on land. This northern manatee lives also near the coasts 

 of Cuba, Haiti, Jamaica and other islands, as well as along the 

 shores of Central America and northern and eastern South 

 America. Curiously enough, a second species (Trichechus 

 inunguis) seems to be confined to the upper reaches of the 

 Orinoco and the river Amazon. Still more remarkable is 

 the fact that aTthrird' species (Trichechus senegalensis) is 

 confined to the coasts and rivers of West Africa, for 

 since the open ocean is to the manatees just as much 

 a barrier to migration as it is to terrestrial mammals, the 

 distribution of these manatees implies the existence of a 

 former shore-line across the Atlantic. It is quite true thait 

 in early Tertiary times manatees have lived much further 

 north than they do now, but the European ones, at any rate, 

 belonged to different genera from those now living. iWe 

 possess no evidence, therefore, for the supposition that the 

 ancestors of the American species passed along the eastern 

 shores of North America and crossed to northern Europe 

 along the ancient Greenland-Iceland land bridge, thus even- 

 tually reaching Africa. Another theory, even less probable 

 I think, is that suggested by Professor Osborn.* He thought 

 a migration might have taken place from Africa by; way of 

 the Pacific coasts of Asia and North America, the ancestors 

 of the West Indian manatees entering the Atlantic through a 

 strait, which is supposed to have connected that ocean with 

 the Pacific, in mid-Tertiary times. He considers this cir- 

 cuitous route a more probable one than the trans -Atlantic one. 

 Yet he does not clearly explain how the close relationship 

 between the West African and eastern South American forms 

 was brought about. 



This, however, by no mean completes all the evidence de- 

 rived from the manatees in favour of the theory of a trans- 

 Atlantic land 'bridge. Dr. Dilg f has pointed out that the 

 molar teeth of the adult living manatees resemble those of 



* Osborn, H. P., "Age of Mammals," pp. 493—494. 



t Dilg, Carl, " Morphologie des Schadels bei Manatus," p. 139. 



