226 



THE AGE OF MAMMALS 



the wide flood plain conditions. A similar fauna (Leptcmchema) was ap- 

 parently partly contained in the lower i)art of the John Day Formation 

 of Oregon, in which, however, fossils are so scarce that reference to the 

 Leptauchenia Zone is provisional. In contrast with the underlying Oreodon 

 beds the 'claj's' are relatively barren. The sparsely preserved plains 

 mammals of the period are contained in the upper part of the Brule Clays 

 or so-called Leptauchenia Zone, while the forest and fluviatile mannnals 

 are found in relative abundance in the Protoceras sandstones, old river 



!.Cf>taurl„;i<„ 





Fig. 112. — Panorama from the summit of Sheep Mountain, South Dakota, an eroded area 

 of the Upper OHgocene. Leptauchenia Zone in the foreground, the underlv-ing Oreodon and 

 Titanotherium Zones appearing in the distance. Photograph by American Museum of Natural 

 History, 1904. 



channels of brilliant greenish tint which traverse different levels of the 

 finer buff and pinkish deposits of the Clays. Vertical or 'organ pipe' 

 erosion of these 'clays' is very characteristic. Whitish layers of volcanic 

 ash occur, and very few fossils are found. 



Remains of mammals are altogether far more scarce than in the under- 

 lying Oreodon Zone, being confined to 21 species. Of these the chief new 

 member is the castorid (Steneofiber) which makes its first appearance here 

 in the form of a very primitive species, S. nehrascensis. This animal be- 

 comes of decided chronologic value in its subsequent evolution because of 

 its wide geographic distribution. 



Highly characteristic of and giving the name to the sandstones is the 

 remarkable artiodactyl Protoceras, an animal of the height of the sheep, 

 remotely related to the American deer and to the hypertragulids, but not 



