1915] 



Clark: Fauna of the San Pablo Group 



435 



Recent molluscan species to the total determinable number of mollusean 

 species in the San Pablo is 23-4-, or about half that of the Etchegoin. 

 A comparison of the percentage of Recent gastropods in the fauna of 

 the Etchegoin with the percentage of Recent gastropods in the San 

 Pablo also shows a striking difference. In the Etchegoin 25 per cent 

 of the gastropods listed by Arnold are Recent. In the San Pablo the 

 percentage of Recent is 11. Only eleven species of the gastropods 

 found in the San Pablo of Middle California are found in the 

 Etchegoin. 



Relation to Santa Margarita. — A survey of the fauna from the 

 Santa Margarita of the type area and of the area north of Coalinga 

 shows that it is much more closely related to the fauna of the San 

 Pablo than to that of the Etchegoin. About one-half of the known 

 species of the Santa Margarita are also found in the San Pablo 

 of Middle California. The species which are common to the San 

 Pablo and the Santa Margarita of the south are forms which are 

 generally recognized as being good horizon determiners, such as the 

 echinoderms, and the more highly ornamented pelecypods and gas- 

 tropods. 



The Santa Margarita north of Coalinga apparently represents 

 only the Astrodapsis whitneyi zone of the San Pablo group of middle 

 California. In the vicinity of Oilfields, the f ossiferous marine beds 

 of the Santa Margarita are a little over 400 feet thick, and Astro- 

 dapsis whitneyi occurs throughout the section. Here it is associated 

 with species common to the Astrodapsis whitneyi zone of the San 

 Pablo of Middle California, such as Venus martini, Dosinia merriami, 

 var. occidentalis, Dosinia arnoldi, Ostrea titan, Pinna alamedensis, Can- 

 cellaria pabloensis, and Trophon carisaensis. None of these have been 

 found in either the Lower San Pablo, or in the Etchegoin. 



The fauna from the Santa Margarita on the west side of the Coast 

 Ranges is as yet imperfectly known. The Astrodapsis whitneyi zone 

 is present and probably beds equivalent to the lower San Pablo of 

 Middle California are also represented. 



Relation to Jacalitos. — The fauna above the Astrodapsis whitneyi 

 zone of the San Pablo of Middle California is not represented in 

 the Santa Margarita section north of Coalinga. This upper fauna 

 may possibly be in part equivalent to that of the Jacalitos of Arnold. 

 This probable relationship is suggested by the presence in the Jacalitos 

 of such species as Astrodapsis, cf. tumidus, large variety, Cyrena (Cor- 

 bicula) californiea, Mulinia, cf. pabloensis, Pectcn, aff. crassicardo, 



