THE EOCENE PERIOD 793 



The non-placental mammals. During the Eocene, early 

 forms of the opossum appeared in both the Old and New World. 

 They retained this wide distribution until the Miocene, when they 

 disappeared in Europe, but they have persisted in North and South 

 America to the present time. It is a singular fact that the mono- 

 tremes, the lowest of the mammals, are not known to have appeared 

 until after the Tertiary. 



The primates (Quadrumana). No traces of apes have been 

 found in the Eocene, but representatives of the lower primates, the 



533. The skull and jaw of a large Eocene rodent, Tillotherium fo- 

 diens Marsh, from the Bridger formation, Wyoming; about Y natural 



lemuroids, appeared in the Wasatch epoch in America, and in a 

 similar horizon in Europe. This is the more notable, as the lemurs 

 are now confined to Madagascar, Africa, and southern Asia. The 

 lemurs show many affinities with the insectivores, and were possibly 

 derived from them. The apes are probably descended from the 

 early lemuroids. 



The mammals go down to sea. Just as the land reptiles of 

 Mesozoic times took to the sea by choice or necessity, so did the 

 mammals in their day. Thus arose cetaceans (whales, dolphins, 

 porpoises), sirenians (manatees, dugongs), and pinnipeds (seals, sea- 

 lions) . In parts of Alabama, vertebrae of primitive whales (Zeuglo- 

 dons) were originally so abundant as to attract popular attention 

 and call forth legends of divers catastrophes. 



