152 PROCEEDINGS OF THE PACIFIC COAST SECTION 



constant characters were defined in the original descriptions. Those of the 

 hinge teeth are the least variable of all of the characters, A nomenclature of 

 the mactroid hinge has been proposed but not generally adopted. The denti- 

 tion of Schizothcerus nuttallii (Conrad) varies in certain minor details, but 

 yet it is more constant than the general form of the shell. Schizodesma 

 al)scissa Gabb. Mulinia densata Conrad, and Pseudocardium gaMi (Remond) 

 are important- forms whose rehitious are imperfectly known. The first is an 

 exceedingly variable species easily confused with MuUnia densata Conrad, but 

 easily separable on hinge characters. Mulinia densata Conrad differs mark- 

 edly from Gabb's Mulinia densata; Pseudocardium gabH (Remond) differs 

 in its dentition from Mulinia densata Conrad as known from the Coalinga 

 field. Forms closely related to these three species should not be recognized as 

 species or varieties unless they are based on hinge characters. 



OBSERTATIOXS OX THE USE OF TEE PERCEXTAGE METHOD IX DETERMIXING 

 THE AGE OF TERTIARY FORMATIOXS IX CALIFORXIA 



BY B. MABTIN 



GEOLOGICAL RELATIOXS BETWEEX THE CRETACEOUS AXD TERTIARY OF 

 SOUTHERX CALIFORNIA 



BY CLABEXCE A. WABIXG 



(Adstf-act) 



An account of the structural and faunal relations between the Chico and 

 Martinez of the Calabasas sheet and a resume of their relations in neighboring 

 territories. 



ECHIXODERMS OF THE SAX PABLO 



BY W. S. W. KEW 



(Abstract) 



In the Tertiary of the west coast, echinoderms are the best horizon deter- 

 miners because of their limited range and their distinctive characters. The 

 San Pablo Series in the vicinity of Mount Diablo has up to the present time 

 shown the best known series of echinoid species. Thus far eight forms have 

 been found in the San Pablo Series. 



FAUNA OF THE SAN PABLO SERIES 

 BY B. L. CLABK 



(Abstract) 



The faima of the San Pablo Series is distinctly Miocene as determined on 

 the basis of the percentage method. The fauna contains about 150 species, less 

 than 2.5 per cent of which are recent. Two distinct major faunal zones are 

 recognizable everywhere ; these, again, are separable into minor zones. From 

 the Lower Major Zone of the San Pablo Series 69 species have been obtained. 

 41 of which have not been found in the Fpper Major Zone. Two minor faunal 

 zones have been recognized in the Lower Major Faunal Zone everywhere that 

 the San Pablo is found. These two minor zones are largely based on the 



