20 



University of California Publications in Geology [Vol.9 



The following species, which are believed to be characteristic of 

 the Oligocene and which are very common in the Oligocene in the 

 north, were originally described from the Agasoma gravidum beds of 

 Contra Costa County: Macrocallista? mathewsonii (Gabb), Mytilus 

 mathewsonii Gabb, Agasoma gravidum (Gabb), Ancillaria fishi Gabb, 

 Bursa mathewsoni (Gabb), Ficus pyruformis Gabb, Molopophorus 

 biplicatus Gabb. Some of these, Macrocallista f mathewsonii, Mytilus 

 mathewsonii, Agasoma gravidum, Molopophorus biplicatus, have been 

 reported in the Lower Miocene (Tnrritella ocoyana and Turritella 

 inezana zones). The writer has had an opportunity to examine most 

 of the forms determined as these species from the Miocene, and with 

 possibly one or two exceptions these determinations are believed to 

 be wrong. 



Much remains to be done before the faunal zones of the marine 

 Oligocene of the west coast are definitely outlined. Arnold and Han- 

 nibal 20 have recognized three faunal zones in their Astoria series — 

 the San Lorenzo, the Seattle, and the Twin River formations. The 

 San Lorenzo horizon is correlated by them with the San Lorenzo 

 formation of the Santa Cruz Mountains in California. Nearly all of 

 the new species originally described by Arnold from the San Lorenzo 

 of California have been found in the San Lorenzo of Oregon and 

 Washington. A number of these are also found in the Seattle beds. 



The fauna of the Lincoln Formation described by Professor C. E. 

 Weaver is considered by Arnold and Hannibal as the equivalent of 

 the San Lorenzo horizon. The Porter beds in the vicinity of the town 

 of Porter, originally referred to the Miocene by Weaver, are at least 

 in part Oligocene and apparently belong to the San Lorenzo horizon. 



Much remains to be done before the faunal zones of the marine 

 Oligocene of the west coast are definitely outlined. For this reason 

 only a very general statement as to the probable position of the 

 Agasoma gravidum fauna in the Oligocene section of the west coast 

 will be made. The fauna of the Agasoma gravidum zone is possibly 

 somewhat younger than that of the San Lorenzo. Certain species 

 which are found in the Santa Cruz Mountains and in Oregon and 

 Washington, and which are thought to be characteristic of the San 

 Lorenzo horizon, have not been found in the Agasoma gravidum beds. 

 Also the lack in the fauna of the Agasoma gravidum beds of certain 

 Eocene species found in the San Lorenzo horizon of Oregon and 

 Washington, is quite noticeable. However, this evidence is negative, 



20 Arnold, Kalph, and Hannibal, Harold, op. cit., p. 579. 



