THE REIGN OF MAMMALS. 213 



animals as in plants, that in the successive periods of the 

 Tertiary, America presents an older aspect than Europe, just 

 as its modern fauna still contains such old forms as the 

 opossum. 



It would seem that as the mountain-ranges and table-lands 

 of Western America emerged from the Cretaceous waters, they 

 became clothed with Eocene forests and inhabited by Eocene 

 mammals. But the waters, dammed up by surrounding ridges, 

 formed large lake basins, which were drained only by the 

 slow excavation of "canons" as the land rose still higher. 

 In the successive deposits formed in these lakes both by 

 ordinary deposition of silt and by paroxysmal showers of 

 volcanic aslies were entombed great numbers of the animals 

 which fed on their banks. It appears that these deposits, 

 which in some places are estimated at not less than 8000 

 feet in thickness, hold the remains of three successive faunas, 

 differing materially from each other, and representing the 

 Lower, Middle, and Upper Eocene. On the flanks of the 

 elevated region supporting the beds formed in the Eocene 

 lakes, are other later lake basins of Miocene age, also 

 abounding in animal remains. East of the Rocky Mountains, 

 and also on the Pacific coast, are still • later Pliocene deposits 

 holding other and more modern Mammalia. The vast area 

 of these formations and the complete sequence which they 

 show are scarcely equalled elsewhere. 



As in the Paris basin, the large Ungulates constitute the most 

 conspicuous feature. This great group is now usually divided 

 into those that are odd-toed (Perissodactyl), and those that 

 are even-toed (Artiodactyl). Though these are apparently 

 arbitrary characters, they correspond with other more funda- 

 mental differences. The first includes such modern animals 

 as the Rhinoceros, Tapir, and Horse. The second includes two 

 somewhat distinct assemblages — that with mammillated teeth, 

 of which the Hog and Hippopotamus are types (Bunodonts), 

 and that with crescental plates of enamel in the teeth, of which 



