306 CLARK AND ARNOLD MARINE OLIGOCENE 



rate tliis fauna from that of the Molopophorus lincohiensis zone. Later 

 work^ may possibly show that here we are dealing with two distinct epochs 

 of deposition. This Acila gettysburgensis fauna also is very different 

 from that of the Molopophorns lincohiensis zone, as well as being de- 

 cidedly different from that of the Agasoma acuminatus beds. However, 

 considerably fewer species are kno^^ai from the Acila gettysburgensis zone 

 than from the Molopophorns lincohiensis zone, and it may be that the 

 difference between the two faunas is more apparent than real. 



CLIMATIC CONDITIONS 



R. E. Dickerson,'' in a recent publication, has expressed the opinion 

 that the fauna of the Acila gettysburgensis zone lived under more tem- 

 perate conditions than that of the Molopophorns lincolnensis zone. We 

 are left to infer from that that very probably the big difference between 

 these two faunas is due to temperature rather than to the time factor. 

 That Dickerson's observations are, at least in part, true is apparently 

 shown by the work of B. L. Clark on the fauna of the San Lorenzo of 

 middle California. Here in the same beds is found a considerable num- 

 ber of species which in Oregon, Washington, and Vancouver Island have 

 been found only in the Agasoma acuminatus beds, the Molopophorns 

 lincolnensis zone, or the Acila gettysburgensis zone, thus showing that 

 apparently all three faunas are more closely related in time than was 

 indicated by the data obtained from the northern localities. Some of the 

 species, which in the northern localities have been found in distinct zones, 

 are found in California associated in the same horizon. As these species 

 are highly ornamented gastro^jods, forms which one might expect to have 

 a rather short life period, they indicate nearly the same, if not identical, 

 age. 



This intermingling in the California localities of certain of the species 

 of the three faunas as recognized in the northern areas, as cited above, 

 suggests very strongly that in the southern localities conditions of tem- 

 perature existed which were intermediate between the temperature condi- 

 tions which existed during the accumulation of the Agasoma acuminatum 

 beds and the beds of the Acila gettysburgensis zone and the more tropical 

 conditions represented by the deposits of the Molopophorns lincolnensis 



^ If this should prove to be the case, the lower. beds would be referred to the San 

 Lorenzo Group, the upper to the Seattle Gi'oup. However, at the rJresent time we do 

 not propose such a classification. The name Acila gettysburgensis zone was first used 

 by Weaver in the paper already referred to, 



8 R. E. Dickerson : Climate and its influence upon the Oligocene faunas of the Pacific 

 Coast, with descriptions of some new species from the Molopophorus lincolnensis zone. 

 Proc. California Acad. Sci., 4th ser., vol. vii, no. 6, 1917, pp. 162-163. 



