104 University of California Publications in Geology [Vol.11 



(See page 95) for complete list of species common to San Lorenzo of 

 Mount Diablo region and Oligocene of Oregon, Washington and Brit- 

 ish Columbia.) A number of these species, such as Acila shumardi, 

 Acila gettysburgensis, Acila muta, Agasoma gravidum, Agasoma 

 acuminatum, Cardium lorenzanum, Macrocallista pittsburgensis, Fusi- 

 nus hecoxi, Fusinus (Exilia) lincolnensis, Turritella diversilineata, 

 Turritella porterensis, Turritella porterensis sobrantensis, and Turris 

 thurstonensis, are forms that are generally recognized as good horizon 

 markers. 



The work on the faunas of the San Lorenzo of the region of Mount 

 Diablo apparently shows that these three general faunas, as recog- 

 nized in the north, the faunas of the Sooke beds, of the Molopophorus 

 lincolnensis zone and of the Acila gettysburgensis zone, are fairly 

 closelj- related in point of time. In the San Lorenzo beds of Mount 

 Diablo are found a number of species, which in Oregon and Washing- 

 ton have been found on\y in the beds of the Molopophorus lincolnensis 

 zone, the equivalent of the San Lorenzo of Arnold and Hannibal, 

 or in the Sooke horizon. Some of the most important of these species 

 are Acila shumardi, Macrocallista pittsburgensis, Agasoma gravidum, 

 Agasoma acuminatum, Fusinus (Exilia) lincolnensis, Turris thurston- 

 ensis, Turritella porterensis, Turritella porterensis sobrantensis. On 

 the other hand, associated with the above named species is a fairly 

 large number of forms, which in the northern localities have been 

 found only in the fauna of the Acila gettysburgensis zone. Some 

 of these species are Acila gettysburgensis, Acila muta, Chione lineo- 

 lata, Miopleiona indurata, and Turritella diversilineata. 



Thus, some of the forms, which have been used as markers of the 

 different faunas as discussed above, are found mixed in a way such 

 as we would not expect if they belonged to distinct geologic horizons. 

 It is also a noticeable fact that over half of the species common to 

 the faunas of the Oligocene of the Mount Diablo region and that of 

 Oregon and Washington are found in the northern localities in all 

 three of the general faunas mentioned above. 



Possibly not enough is yet known concerning the range of the 

 different species in the marine Oligocene of the West Coast to defi- 

 nitely place the position of the San Lorenzo fauna as found in 

 California, in reference to the northern sections. The writer is of 

 the opinion, however, that this fauna is probably somewhat older than 

 that of the Acila gettysburgensis zone, the uppermost Oligocene of 

 Oregon and Washington, and that very possibly it is not as old as the 



