6 



gape ; posterior end of valves squarely rounded ; shell dull chalky white, sculp- 

 tured in concentric lines, which anteriorly are laminated and posteriorly become 

 extinct ; valves radiately ribbed, which also become obsolete at the posterior 

 end ; at the intersection of the radiating and concentric lines the sculpture is 

 pectinated ; an area below the umbos nearly or quite destitute of sculpture, 

 which varies much in prominence in different specimens ; accessory plate sub- 

 lanceolate and bent down on the beaks, anteriorly prolonged, but not wholly 

 covering the aute-umbonal gape ; figs. 6a, 6b, show the variation in the shape 

 of the dorsal plate in different specimens ; interior of valves white, enamelled ; 

 internal rib short, curved and flattened. Largest specimen, two and six-tentlis 

 inches in length, and one and five-tenths inches in height. 



Habitat. — Alameda, San Francisco Bay, California, where in some places it 

 is common in sandy mud between tide marks. Numerous specimens collected 

 by Messrs. Harford, Hemphill, Drs. Kellogg and W. P. Gibbons. 



This shell is the West Coast analogue of the Atlantic P. truncata, Say, which 

 it resembles ; it is however a much longer shell for its width, and the portion of 

 the valves posterior to the beaks, very much longer than in Say's species. Spec- 

 imens of this species have been distributed as Zirphaa crispata, which also is 

 found upon the coast, though quite distinct from P. Facifica, which latter comes 

 within Mr. Tryon's subgenus Cyrtopleura. 



According to the Messrs. Adams in the genus Pholas, there are two dorsal 

 plates ; yet they have included in their list of the species under that genus, P. 

 truncata, Say, which has only one. 



