308 CLARK AND ARNOLD MARINE OLIGOCENE 



tlie same general areas. We find the same difficulty in evaluating the 

 causes of the differentiation of the Molopophorus lincolnensis fauna and 

 that of the Acila gettyshurgensis zone. 



We may say with certainty, however, that during the Oligocene on the 

 West Coast we find the beginning of conditions like those which existed 

 during the Miocene and Pliocene, a beginning of the differentiation of 

 the climatic zones, which were not defined during the Eocene, and with 

 it the local differentiation of faunas, due to temperature harriers — a con- 

 dition which did not exist to any appreciable extent during the Eocene. 



