MARINE 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 a third 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 that 
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. We 
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.* ITe 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, II. F., “Age of Mammals,” pp. 493—494. 
+ Dilg, Carl, “ Morphologie des Schadels bei Manatus,” p. 139. 
