1915] 



Clark: Fauna of the San Pablo Group 



439 



and Pliocene of Europe and the east coast of North America has 

 been, at least approximately, established on the basis of the percentage 

 method. 



The percentage of Recent molluscan species in the San Pablo of 

 Middle California as listed by the writer is 23+ ; as based upon the 

 gastropods the percentage is only 11 per cent. If we use the per- 

 centages as applied to the east coast Neocene and if we can rely upon 

 the equal refinement in the determination of the species, the San Pablo 

 may be considered to be Upper Miocene in age, possibly Lower Pliocene. 



Accepting this determination of the age of the San Pablo, the 

 Etchegoin and other formations which are equivalent to it must of 

 necessity be Pliocene in age. This determination is borne out by the 

 high percentage of Recent species in the latter horizon. 



As yet no satisfactory correlation has been made between the west 

 coast and the east coast of North America or with Europe on the 

 basis of identity of invertebrate species. The marine molluscan fauna 

 of the east coast of North America, except for a few circumboreal 

 species, is almost entirely different from that of the west coast. In 

 general it may be said that for every coast of the different continents 

 there is a distinct molluscan faunal province and on each coast there 

 are distinct subfaunal provinces. 



The great differentiation in molluscan faunas which we find at 

 the present day apparently also existed during the Neocene. Very 

 few species, if any, of the marine molluscan faunas of the Neocene 

 of the west coast are found in the Neocene of the east coast or of 

 Europe. The difficulty in correlating the Neocene of different parts 

 of the west coast, as between Washington and California, also shows 

 that local faunal provinces existed then as at the present time. 



The same factors which have brought about the differentiation of 

 the molluscan faunas of the Neocene and Recent over a large part of 

 the world have produced the generalization of the land vertebrate 

 faunas. The close connection of the great continental masses during 

 the Neocene, separating seas which had previously been united, to- 

 gether with the well-marked climatic zones, isolated the invertebrate 

 faunas of the different coasts and therefore brought about theii 

 differentiation. This allowed a wide migration of the vertebrate 

 forms, the faunal provinces being enlarged so as to include more than 

 one continent. 



From the above facts it would seem self-evident that if determin- 

 able land vertebrates could be found in the west coast marine Neocene 



