232 



THE AGE OF MAMMALS 



conditions of deposition are more fully discussed under the Miocene, 

 p. 278. 



As soon as fossils become abundant it is evident that we are in the 

 Promerycochoerus Zone, because, while the entire assemblage of mammals 

 is, on the whole, similar generically to that of the Upper John Day, the 

 specific stages are regarded as type for type more recent than those either 

 of the Middle or even of the Upper John Day, or of the Aquitanian or St. 

 Gerand-le-Puy of France. The peccaries afford similar testimony. Thi- 

 nohyus siouxensis is more modified than any of the species of the John 

 Day.^ It is interesting to note that these peccaries, like those of the John 



Fig. 116. — Upper John Day Formation, Oregon ; Promerycochoerus Zone. Photograph by 

 the University of California expedition of 1900. 



Day, fall into two phyla, a dolichocephalic and a brachycephalic. This more 

 recent character and the presence in these lower Arikaree deposits of the 

 plains of several new genera of mammals may justify the placing of these 

 formations in the Lower Miocene (Peterson, Matthew). We are certainly 

 in the presence of a transition. 



As compared with the Old World, however, it would appear that this 

 mammalian assemblage of the Upper John Day, Lower Arikaree, Lower 

 Harrison, and Lower Rosebud is still characteristically Oligocene rather 

 than Miocene, as shown in the accompanying list. 



1 Peterson, O. A., New Suilline Remains from the Miocene of Nebraska. Mem. Carnegie. 

 Mus., Pittsburg, Vol. II, no. 8, 1906, pp. 305-320. 



