210 



THE AGE OF MAMMALS 



tortoises (Testudinata), as analyzed by Hay/ furnish important proof of 

 prevailing dry land conditions on the great plains. How long previously 

 such conditions had set in it is impossible to say. In the entire Oligocene 

 and Miocene beds of the great plains only six species of water-living turtles 

 have thus far (1907) been recorded, and these are probably from river 

 channel sandstones, as contrasted with a very much larger number of 

 land-living tortoises, chiefly from fine clay deposits. The upland testudi- 

 nates include in the White River group (Lower to Upper Oligocene) eight 

 species of land tortoises {Stylemys, Testudo). Remains of crocodiles have 

 been recorded (Loomis) - in river channel beds of Lower Oligocene age. 



Physiographic conditions. — The general conditions of Oligocene life in 

 the plains region have already been pictured in the early part of this chap- 

 ter (p. 179), and we may now review the characters of each subdivision of 

 the Oligocene more in detail. 



Lower Oligocene, Lower White River, or Chadron Formation, 



TiTANOTHERIUM ZoNE 



This takes us at once into one of the grandest and most famous of 

 mammal-bearing horizons, the 'Titanotherium Beds' of Leidy and Hayden, 





^^^,^ in fin 



Fig. 98. — In the ' Big Badlands ' of South Dakota ; Lower and Middle Oligocene. Lower : 

 Titanotherium Zone including channel beds, a river formation. Upper : Oreodon Zone, a 

 flood-plain. Photograph by American Museum of Natural History, 1907. 



^ Hay, O. P., The Fossil Turtles of North America. Publ. Carneg. Inst., Washington, 

 no. 75. 4to, 1908. 



' Loomis, 1904, op. cit 



