306 University of California Publications in Geology [Vol. 9 



the Pseudocardium form, but the hinges of the two are to all essen- 

 tials identical. 



The differences of the hinge of the left valve, as figured on plate 

 29 are those of varying thicknesses of the laterals and the arms of 

 the cardinals. This thickening appears to be a function of the 

 shortening of the hinge line of the more ventricose forms. These 

 dental characters grade insensibly along with the changing shape of 

 the shell from that of the elongate typical form with its long laterals 

 to the Pseudocardium form with its robust teeth. 



It is worthy of mention that in the region of Mount Diablo, 

 California, the great majority of specimens belong to the ventricose 

 type, whereas at a higher horizon in the Coalinga District these 

 Pseudocardium forms are the exception. This might suggest that 

 these two forms represent two parallel races, one of which became 

 dominant during the San Pablo, while the typical form reached its 

 maximum in the Etchegoin ; and that they did not diverge enough 

 from the parent stock to be easily classified as definite species. 



The hinge plate of Mulinia densata is rather wide; chondophore 

 deep ; roofed apically and overhung by a lamella in the left valve ; 

 the left cardinal prominent, often heavy, the arms of equal width, 

 and often fused together in the Pseudocardium forms, producing a 

 triangular block-tooth and varying in this respect to a condition 

 where the arms are clearly defined ; the arms of the cardinal reach 

 ventrally nearly across the hinge plate ; the anterior arm of the 

 right cardinal is more prominent than the posterior one ; laterals of 

 unequal length; distant from the beaks, being relatively short and 

 heavy in the more ventricose forms; anterior lateral nearly in align- 

 ment with the cardinal arm, but distant from that tooth; laminae 

 heavy, the anterior ventral one is in line with the cardinal arm; 

 dorsal laminae small, arising from the dorsal margin of the shell. 

 Pallial sinus shallow, broadly rounded. Mulinia densata is separable 

 from M. pabloensis by the lack of striations on the lateral teeth ; from 

 M. alta by its much heavier left cardinal and wider hinge plate; 

 and from M. undilifera by its narrower hinge plate and more deli- 

 cate teeth. 



Conrad's type was obtained from the vicinity of Santa Bar- 

 bara, California. 



Occurrence. — Pliocene : Etchegoin. Miocene : San Pablo, Upper 

 Monterey. 



