208 THE AGE OF MAMMALS 



and not mountainous, l)earing Ijroad streams with varying channels, some- 

 times spreathng into shallow lakes, but never into vast freshwater sheets. 

 Savannahs were interspersed with grass-covered pampas traversed by broad, 

 meandering rivers. This land was dry in dry seasons, but was flooded in very 

 high water periods. The materials were partly erosion products of the Rocky 

 Mountains and Black Hills, such as true sandstones and conglomerates, but 

 they included also fine layers of volcanic dust, wind-borne from distant 

 craters in the mountains, far out on the plains of Nebraska and Kansas. 



Scattered through these Titanotherium and Oreodon beds^ are numer- 

 ous thin layers of limestone, always of limited areal extent, rich in remains 

 of freshwater plants (Chara) and molluscs {Limncca, Physa, Planorhis), of 

 species inhabiting swamps and small ponds. Remains of forests are found 

 at different horizons throughout these beds, including silicified trunks of 

 trees and seeds belonging especially to forest types {Hickoria, Celtis). 

 Nothing like complete trunks are observed, and the impression was that 

 of burial on stream margins where only the less destructible parts of trees 

 would endure sufficiently long to be covered up and preserved. (Hatcher.) 



Prevailing mammal types. — It will be observed that in the above de- 

 scribed Oligocene mammals of Avestern and central Europe, there are no 

 plains- or upland-living types; horses are absent, the hornless deer-like 

 forms are the gelocids and cervulines, analogous to those now frequenting 

 swampy or forested regions. The dry ground or upland fauna, if it existed, 

 has not been discovered. 



In America, on the other hand (see p. 220), ])oth the low ground and 

 the high ground mammals of the Oligocene are known, the former liroadly 

 agreeing in foot and tooth structure with those of Europe; the latter, in- 

 cluding the horses and camels, are fleet, cursorial types. Thus the physiog- 

 raphy of the plains country was varied. 



As this is the first glimpse of the life of the great plains of America, 

 it is probable that many of the mammals which are found here were not 

 new to North America, but had been resident on the Great Plains for a 

 considerable period. 



Oligocene lizards? — Indications of dry land conditions in the Titano- 

 therium and Oreodon zones of Montana are found in the presence of numer- 

 ous lizards of a type (Glyptosaurus) which has the skull covered with tuber- 

 culated bony plates. These animals are referred to the burrowing, nearly 

 limbless family of Anguidae, and are related to forms also found in the 

 Eocene of the Bridger Formation of Wyoming. In the Oligocene of Ne- 

 braska the worm-like, amphisbsenian lizards (Rhineura, Hyporhina) occur, 

 animals now inhabiting the tropical regions of America and Africa. 



In addition to the evidence drawn from geology and the mammals, the 



' Hatcher, J. B., Origin of the OHgocene and Miocene Deposits of the Great Plains. 

 Proc. Anier. Philos. Soc, Vol. XLI, 1902, pp. 113-131. 



- Douglass, E., Some Oligocene Lizards. A?in. Carneg. Mas., Vol. IV, nos. 3 and 4, 1908, 

 pp. 278-285. 



