1915] 



Clark: Fauna of the San Pablo Group 



453 



the valves have not been too much eroded, tend to come to a point on 

 the tops of the ribs and riblets. The larger ribbing becomes finer 

 toward the dorsal margins while the riblets become more prominent, 

 the last few ribs not being distinguishable from the riblets ; sometimes 

 there are six or seven or even more of these finer undifferentiated ribs 

 near the margins. The ribbing on the larger specimens is broader and 

 flatter and tends to be dichotomous toward the ventral edges; this 

 splitting of the ribs first makes its appearance toward the outer edges, 

 but on the larger shells all the ribs may be dichotomized ventrally; 

 in some cases the riblets also are dichotomous. Anterior ear deeply 

 notched, sculptured by four to six fairly prominent radiating ribs. 

 Margin of posterior ear straight, sculptured by three to six medium 

 fine, radiating ribs. Left valve usually more ventricose than right 

 valve ; sculptured by seventeen to nineteen low, rounded radiating ribs 

 with the interspaces nearly twice as wide as the tops of the ribs; a 

 fine narrow riblet in each interspace as on right valve; ribbing finer 

 toward dorsal margins, becoming indistinguishable from riblets ; the 

 larger ribs as in the case of the right valve tend to become dichotomous 

 and the riblets also tend to split. Surface imbricated as on right 

 valve. Anterior ear slightly notched, sculptured by four to six medium 

 fine radiating ribs ; posterior ear with about equal number of ribs, 

 posterior edge straight. Ligamental groove fairly heavy. Resilifer 

 small. Adductor muscle impression large. 



P. pabloensis is found associated with P. bilineatus, P. liolwayi 

 and P. crassicardo. It has been confused many times with P. bil- 

 ineatus and P. raymondi. In Professional Paper 47, pi. XXVII, 

 fig. 5, Arnold has figured the left valve of this species. Figure 6 of 

 the same plate represents a specimen of P. bilineatus, while figure 7 

 appears to represent the left valve of P. raymondi, n. sp. 



Dimensions. — Specimen figured on plate 47, figure 8. Height, 32 

 mm. ; length, 39 mm. ; length of hinge, 22 mm. ; length of anterior 

 dorsal edge, about 18 mm. ; length of posterior dorsal edge about 22 

 mm. ; diameter of right valve, about 3 mm. 



Occurrence. — On San Pablo Bay in the Upper San Pablo group. 

 University of California localities 1632, 1613, 1614 and 1615 ; also, on 

 the north side of Mount Diablo in the Upper San Pablo, University of 

 California locality 1891. 



