OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 231 



of specific forms. Similarly the suillines, or dicotylids, are found in great 

 numbers (Perchoerus) and in a variety of specific forms. Of the oreodonts 

 the Agriochoerus phylum reaches a climax preceding its extinction. The 

 typical cursorial oreodonts pass into the Eporeodon stage. The hyper- 

 tragulids are represented only by Hypertragulus. 



Third and Final Phase, Promerycochoerus Zone 



Upper John Day of Oregon. — Especially noteworthy in the Upper 

 John Day is the sudden appearance of Promerycochoerus, an unheralded 

 oreodont, which is highly characteristic of the close of the Oligocene and 

 opening of the INIiocene of North America. This Upper John Day level 

 also contains members 

 of the camel series 

 (Paratylopus) , animals 

 of larger size, and 

 similar to those in the 

 Middle Oligocene. It 

 is noteworthy that the 

 only camels from the 

 John Day o])tained 

 by .the University of 

 California expeditions 

 come from this upper 

 level. 



Great Plains of 

 Dakota, the 'Harrison^ 

 and ' Rosebud. ' — While 

 the John Day Forma- 

 tion was gathering in 

 Oregon, either a con- 

 tinuous but non-fossil -bearing deposition or a brief erosion interval followed 

 on the Great Plains of Dakota, and the great formations variously known as 

 'Arikaree,' 'Gering,' 'Monroe Creek,' 'Harrison,' and 'Rosebud' began to be 

 deposited conformably or unconformably on the summit of the White River 

 group, or Leptauchenia Zone (Fig. 96). This great formation was recog- 

 nized as Horizon D by Hayden as early as the year 1869. It is indicated 

 by the horizontal lines in the map on p. 211, and is extensively exposed 

 along the White, Niobrara, and North Platte rivers. It opens with partly 

 flood plain or fine deposit conditions, interspersed with partly fluviatile or 

 channel conditions, which are indicated liy broad beds of conglomerates 

 and river sandstones (Gering and Monroe Creek formations). The latter 

 are non-continuous; they reach a maximum thickness of 200 feet, and 

 are generally unfavorable for the preservation of mammalian life. The 



Fig. 115. — Type of the Promcrycoch(Tprus Zone. Skull of 

 the Upper Oligocene oreodont Promerycochoerus macrostegus. 

 In the American Museum of Natural History. After Matthew. 



