LIFI-; 



hoof i-d types, and that the latter arose, either once or at several 

 separate times, from the former. 1 



Edentates, rodents, and insectivores. The similarity of the 

 aiut-xiral edentates to the condylarths and creodonts of the earliest 

 Kneene seems to imply that the three orders had but recently di- 

 verged from common ancestors. By the middle of the period, 

 rodents became a notable element in the fauna. The squirrel 

 appeared in Europe in the latter part of the period. Even to-day, 

 the rodents retain many primitive characters, and since the Miocene 

 have undergone few radical changes. Their derivation is not yet 

 determined. Most 

 living families of in- 

 sectivores can be traced 

 back to the Eocene. 

 They still retain many 

 primitive characters, 

 and are theleast altered 

 of the great mamma- 

 lian branches. 



Non-placental mam- 

 mals. During the 

 period, opossums ap- 

 peared in both hemi- 

 spheres. They retained 

 this wide distribution until the Miocene, when they disappeared 

 from Europe, but they have persisted in North and South America 

 to the present. It is a singular fact that the monotremes, the lowest 

 of the mammals, are not known until after the Tertiary. 



The primates. No traces of apes have been found in the Eocene, 

 but 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. They 

 have many affinities with the insectivores, arid were possibly derived 

 from them. Apes probably descended from the early lemuroids. 



Mammals go down to sea. Some mammals took to the sea by 

 choice or necessity, as land reptiles did before them. Thus arose 

 cetaceans (whales, dolphins, porpoises), sirenians (manatees, du- 

 gongs), and pinnipeds (seals, sea-lions). In parts of Alabama, verte- 

 brae of primitive whales (Zenglodons) were originally so abundant 



1 A History of Land Mammals in the Western Hemisphere. 



Fig. 480. The skull and jaw of a large Eocene 

 rodent, Tillothcriitmfodicns Marsh, from the Bridgi-r 

 formation, Wyoming; about 1/6 natural size. 



