378 



University of California . 



[Vol. 3. 



In addition to the shale fauna, there are in Contra Costa 

 County two faunal zones in the Miocene, one above and one 

 below the shale. The upper division has its nearest affinities 

 with the San Pablo, from which it can lie distinguished by the 

 presence of Clypeaster (?) brewerianus, Troc-Mta costellata, sev- 

 eral new species of Modiola, and other forms. 



The fauna of the lower division is much more characteristic 

 than the upper: that is to say, it differs more decidedly from that 

 of the beds immediately above and below it. This fauna includes 

 the following forms: 



Agasoma gravida Gabb. 

 Chrysodornus n. sp. 

 Ficula n. sp. 

 Olivella pedroana Conrad. 

 Neverita near callosa Gabb. 

 Turriiella indet. 

 Crepidula indet. 

 Dosinia mathewsoni Gabb. 

 Chione mathewsoni Gabb. 

 Mytilus mathewsoni Gabb. 

 Venus n. sp. 



Glycimeris near generosa Gould. 

 Leda taphria Dall. 

 Nucida divaricata Hinds. 

 Area n. sp. 



The most characteristic species are Agasoma gravida, Dosinia 

 mathewsoni, Chione mathewsoni, and Mytilus mathewsoni . These 

 beds rest upon the Tejon, which is very fossiliferous only a few 

 yards below the contact. A large percentage of the species in 

 this horizon do not appear in the upper Miocene and as yet not 

 a single species has been found to extend down into the Tejon. 

 In Contra Costa County the fauna of this zone is more, distinctive 

 as that of the Monterey shale, considering even that the latter 

 represents a deep water facies. 



The lower fauna is recognized at many places in Contra 

 Costa County, and always next the base of the series. This 

 faunal division might appropriately be called the zone of Agasoma 

 gravida. Forms like A. gravida have a wide geographic range 

 on this Coast and are apparently characteristic of the base of the 

 Miocene. 



