OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 229 



tions can be made from them except that they are closely related to those 

 of the overlying 'Middle John Day.' 



Our full knowledge of the mammalian life of this great basin, there- 

 fore, begins with the richly fossiliferous levels of the Middle John Day, 

 or Diceratherium Zone, which contains a varied mammalian fauna con- 

 siderably more advanced than that of the Leptauchenia-Protoceras Zones, 

 above described, and, as pointed out by Scott, on the whole closely similar 

 in its evolution to the St. Gerand-le-Puy fauna of France. One hundred 

 species of mammals have- been described from this level, and while this 



Fig. 114. 



Middle John Day Formation, Oregon ; Diceratherium Zone. Photograph by 

 University of California, 1900. 



list may be reduced by closer comparison, it also will be expanded by 

 further exploration. As in the Upper Oligocene of France we note the 

 entire disappearance of the archaic hyaenodonts; among rhinoceroses the 

 cursorial hyracodonts are absent; among Artiodactyla no traces have 

 been found of the primitive family of leptochcerids, nor of the anthraeo- 

 theres; among smaller mammals it is noteworthy that there are no traces 

 of the opossums (didelphyids). 



Among rodents we note the reappearance of the castorid Steneofiber, 

 first observed in the somewhat older Protoceras Zone, and also highly 

 characteristic of the European Upper Oligocene. Still more striking is the 

 first appearance of the peculiarly American hai)lo(lontids or sewellels, 

 which even at the present time are confined to the Rocky Mountain region. 

 These animals (see p. 534) are sciuromorphs, remotely related to the Eocene 



