8 



OSBORN:. OLIGOCENE, MIOCENE, PLIOCENE EQULDtE. 



CHIEF GEOLOGIC HORIZONS, FORMATIONS, LEVELS, AND LIFE ZONES CONTAINING 



REMAINS OF EQUIDjE. 1 



Fig. 4a. Inferior Molar Pattern of Equus niobrarensis Hay, to be compared with that of 



Pliohippus leidyanus Osborn. 

 A Equus niobrarcnsis Hay, Univ. Nebraska Coll. 16-12-13, referred specimen, left lower jaw containing p3-m 3 

 (reversed in drawing). Locality, Crete, Nebraska, attributed to Aftonian Interglacial stage. 



B Equus caballus, Amer. Mus. Coll. 4th inferior premolar of the right side. Selected as the most elongate, hypso- 

 dont, molar in a very large collection. 



Geologic correlation. The geologic correlation in this Memoir is provisional because in many of the formations, beds, 

 and horizons the fauna is imperfectly known or imperfectly determined at present. 



The chief departure from the previous geologic correlations by Osborn and Matthew 1 is in the lower and upper 

 boundaries of the Miocene. The lower boundary of the Miocene is now made to include the Promerycochcerus- 

 Miohippus zone of the Upper John Day, the Lower Harrison, and the Lower Rosebud. The Upper Miocene is made 



1 Osborn, H. F. and Matthew, W. D. "Cenozoic Mammal Horizons of Western North America, with Faunal Lists of the Tertiary 

 Mammalia of the West," Bull. U. S. Geol. Surv. 361, Jan. 1909, pp. 1-138. 



