MAMMALIA. 767 



divisions which actually appear for the first time are few, for nearly all 

 the characteristic divisions had very few and insignificant repi'esentatives 

 during the Eocene. The Artiodadyla ruminantia and the carnivora are not 

 known from the Eocene, while the Artiodadyla omnivora had very few rep- 

 resentatives. The Menodontidce and Palceotheriidce chiefly belong to this 

 epoch, with several families of Rodentia. The Chiroptera, probably the 

 Marsupialia, and the rodent family of the Squirrels held over from the 

 Eocene, while the Proboscidea had not yet appeared. 



No species of Mammal is common to this epoch and any of those of 

 the Eocene, and only one or two genera of Marsupial or Marsupial-like 

 families have continued from the one to the other, so far as present infor- 

 mation extends. 



The species of the White River Epoch attained a larger average size 

 than those of the Eocene Epochs. This is especially evident on comparison 

 of related or corresponding types of the two periods. The Ambhjpoda, 

 which embraces the largest Mammalia of the Eocene, became extinct. The 

 most notable increase of size is seen in the Perissodadyla, where the succes- 

 sion is most continuous. The same is generally true of the flesh-eating 

 series. 



The number of individual mammals of this epoch in Middle North 

 America was evidently very great. Many of the species were represented 

 by great droves, and their bones form beds of considerable extent. A 

 locality in Colorado, examined by the writer, embraced about forty acres 

 of naked, soft calcareous rock, carved by erosive action into areas of various 

 sizes. Here the surface of the rock was found to be covered with the 

 remains of the smaller and some of the larger species of the fauna. There 

 were innumerable rodents, and small Artiodadyla and Carnivora; numerous 

 Marsupialia, Creodonta, and Poebrotherium. There was a great abundance 

 of Hyracodon, and several other rhinoceroses, with quantities of three-toed 

 horses. No traces of the huge Menodontidce were there, but at a locality 

 some miles distant, a similar deposit of these large animals was found, 

 mingled with rhinoceroses. Evidently the causes which overwhelmed the 

 smallei- forms did not aff"ect the giants, which only yielded to some other 

 and more irresistible influence. 



