GEOLOGIC HORIZONS AND LIFE ZONES. 



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6. Cedar Creek beds, 1 northeastern Colorado, Matthew, 1901. 



7. Hat Creek beds, Wyo., Reed, Hatcher. 



8. Exposures of similar age in Wyoming, South Dakota, northwestern Nebraska, western Montana, usually 

 described as "White River beds," "White River Group." (Exact geologic level uncertain.) 



Type of Mesohippus exolctus Cope, 1874.1. Type of Mesohippus cuneatus Cope, 1873.1. 



?Type of Mesohippus eulophus Osborn, 1904.6. 



Bear Creek, Pennington Co., South Dakota, Culbertson, 1850. 



From the published field notes of Thaddeus Culbertson (Fifth Ann. Rept. Smiths. Inst., 1850) and from a later report 

 by Hayden (Proc. Acad. Nat. Sci. Phila. 1857, p. 151), Gidley is of the opinion (letter, October 20, 1917) that all the col- 

 lecting of the early expeditions in the "Mauvaises Terres of Nebraska" was confined to a single area about five miles 

 in extent, near the head of Bear Creek, about eight or ten miles north of Sheep Mountain of the Big Badlands, and that 

 none of these expeditions ever entered the afterward famous Indian Creek-Corral Draw Basin locality, now known as the 

 Big Badlands. The type locality of Mesohippus bairdii and its associated fauna can probably be stated as the head of 

 Bear Creek, Pennington County, South Dakota, from which Leidy noted that several tons of material were taken by the 

 early expeditions. 



Type of Mesohippus bairdii, 1851. 



Lower Oreodon Zone. 



Neotype of Mesohippus bairdii Leidy, 1851. 



Metamynodon Zone (=lower levels of Middle Oreodon Zonej. Stampian, Oligocene Moyen of Europe. 



1. Metamynodon sandstones, South Dakota, Wortman, 1893. River channel sandstones in Lower Oreodon zone or 

 lower Brule Clay, all continuous. 



Type of Mesohippxis trigonostylus Osborn, 1918. Paratype of M. trigonostylus Osborn, 1918 



Middle Oreodon Zone (= level of Metamynodon Zone). Stampian or Oligocene Moyen of Europe. 



Type of Mesohippus obliquidens Osborn, 1904.5. 



Upper Oreodon Zone. 



1. Upper Oreodon layer 75-100 ft., Wortman, 1893. 



2. Similar levels at Cedar CreeK, • Col., Hat Creek, Wyo., and other exposures in South Dakota and northwestern 

 Nebraska. 



? Type of Mesohippus eulophus Osborn, 1904.6. 



Protoceras Zone and Leptauchenia Zone, contemporaneous early phases of Upper Oligocene. 



1. Horizon C (in part), South Dakota, Hayden and Leidy, 1S69. 



2. Upper part of White River group of South Dakota, Meek and Hayden (in part), 1869. 



3. Leptauchenia clay layer (= lower and upper levels), Wortman, 1893. Plains fauna. 



1 Matthew observes (1916) that it is quite likely that a large part of the fossiliferous areas of the Cedar Creek beds are Upper Oreo- 

 don zone and that M. eulophus may be from those upper layers, but there is no positive record of it. 



