10 



OSBORN: OLIGOCENE, MIOCENE, PLIOCENE EQUIDyE. 



Lower Titanotherium Zone. 



Type of Mesohippus celer Marsh, 1874. Type of Mesohippus latidens Douglass, 1904.1. 



" " " westoni Cope, 1889. ? " " " portentus " 1908. 



a u 



" montanensis Osborn, 1904.2. 



? Titanotherium Zone — Leptauchenia Zone. 



Type of Mesohippus prcecocidens Lambe, 1905.1. Type of Mesohippus planidens Lambe, 1905.4. 



propinquus " 1905.2. ? " " " assiniboiensis Lambe, 1905.5. 



Upper Titanotherium Zone. 

 Type of Mesohippus hypostylus Osborn 1904.3. Type of Mesohippus proteulophus Osborn 1904.4. 



Fig. 6. Oligocene (Cedar Creek), Middle Miocene (Pawnee Creek), and Lower Pliocene (Upper Pawnee Creek) 

 exposures of northwestern Colorado. Areas explored by the American Museum of Natural History in 1898, 1901, 1902, 

 by Messrs. Matthew, Brown, and Thomson. 



Oreodon-Mesohippus Zone. Stampian, Olioocene Moten op Europe. 



1. White River group (in part), Meek and Hayden, 1862. 



la. Bear Creek, Pennington County, South Dakota, 1850. 



2. Horizons B and C, South Dakota, Hayden and Leidy, 1869. 



3. Oreodon beds of Leidy, South Dakota, 1869. 



4. Middle portion of Big Badlands, Cheyenne River, South Dakota. 



5. Brule Clays, lower levels, South Dakota, Darton, 1899. 



