258 HEXRY F. OSBORN 



The zonal type is a large titanothere with well-developed bony horns, 

 transitional in many characters to the Lower Oligocene titanotheres; 

 in fact, the summit of the thick Diplacodon Zone of 600 feet will prob- 

 ably prove to coincide with the base of the White River Group on the 

 great plains. Quite recently, during the summer of 1909, the much- 

 desired sequence of Oligocence and Eocene strata was discovered by 

 Mr. Granger of the American Museum expedition. The Diplacodon 

 Zone has been discovered in the Wind River region of Wyoming 

 underlying the Titanotherium Zone. 



The Ligurian stage of France is that of the famous Gypse de 

 Montmartre discovered by Cuvier, full of paleotheres and anoplo- 

 theres, a mammal fauna totally distinct from that of the Rocky 

 Mountain region. 



OLIGOCENE 



Lower. — The Oligocene opens in the New and Old Worlds with 

 the fourth faunal phase and second modernization, which since it 

 affects alike Europe and America probably indicates a fresh migration 

 from the great unknown northern, or Holarctic region. With this 

 migration close faunal resemblance is re-established with western 

 Europe, and thereby comes a welcome means of geologic correlation; 

 in other words, we may with considerable confidence consider that 

 the base of the White River group was nearly coincident with the 

 inferior Tongrian of France. Sixteen new families of mammals 

 appear in America, all of them still existing, and seventeen modern, or 

 still existing, families appear in Europe. This momentous faunal 

 change in North America is partly attributable to the fact that this is 

 our first glimpse of the life of the Great Plains. 



Middle. — The Lower Oligocene, or Titanotherium Zone, most 

 accurately surveyed by Hatcher, is succeeded by the Middle Oligocene 

 or Oreodon Zone, broadly comparable with the Superior Tongrian 

 and Stampian of France, both containing similar types of amphibious 

 rhinoceroses and many other mammals. One of the chief points of 

 interest here is the sharp separation discovered by Matthew between 

 the plains-living mammals buried in the so-called clays, or finer 

 deposits, and the forest-living mammals buried in the coarser intrusive 

 river sandstones. 



