OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 237 



By pennlsslon of E. H. Barbour. 



Fig. 121. — Skull of the four-pronged ruminant Syndyoceras, found in the Dsemonelix beds of 

 Sioux County, Nebraska. Univ. Nebr. Coll. 



either: (1) Upper Oligocene, or (2) transitional from Oligocene to Miocene, 

 or (3) of pure Lower Miocene age (see p. 277). 



Causes of Extinction of Oligocene Mammals' 



In considering Eocene extinction we have found (p. 172) the note- 

 worthy feature in America to be the competition, or struggle for existence, 

 between lower and higher types of mammals, or the archaic and modern- 

 ized, as a result of which the former entirely disappeared. 



In Europe this feature is less conspicuous than the apparent influence 

 of the altered land areas of a peninsular region with the large number of 

 consequent changes (p. 83). This is the more apparent in Europe because 

 the archaic mammals do not appear ever to have had such a strong foot- 

 hold in Eocene times as in continental America. We note the disappear- 

 ance of a very large number of prophetic forms of modernized mammals 



' Os})orn, H. F., The Causes of Extinction of Mammalia. Amer. Natural., Vol. XL, 

 no. 479, Nov., 1906, pp. 709-795, no. 4S0, Nov., 1906, pp. 829-859. 



