18 PALAEONTOLOGY OF CALIFORNIA. 



ZIRPH^EA, Leach. 



Z. DENTATA, n. S. 

 PI. 3, Fig. 31, 31 a. 



Shell large, subcylindrical, thin; ends broadly gaping; beaks 

 anterior to the middle ; covered by the dorsal plate ; posterior 

 dorsal margin of valves thin and sharply reflexed. Surface of 

 anterior third of shell marked by serrated, squamose plates, as 

 in Z. erispata; a faint line or rib passes between the middle and 

 posterior third of the width, from the beaks obliquely down to 

 the base; dorsal plate heavy, compressed, and divided into two 

 concave surfaces by a sharp, angular ridge, commencing at the 

 posterior end, and running forwards, slightly curved, ending in 

 a tooth at a point about a third of the length of the plate from 

 the anterior extremity. 



Figures, natural size ; fig. 31 a, represents the dorsal plate. 



Locality: A single specimen from the Pliocene beds at the east end of Kirker's 

 Pass, Contra Costa County. 



This shell is closely allied to Z. erispata; hut differs in its more regularly cylin- 

 drical form, the marked reflection of the posterior dorsal margin, and the angular 

 dentated dorsal plate. 



PANDORA, Brug. 



P. SCAPHA, U. S. 

 PL 4, Fig. 32. 



Shell small, irregularly subquadrate; right valve flat; left 

 valve slightly convex; beaks small, about a third of the width 

 from the anterior end, which is prominent and regularly convex; 

 cardinal margin slightly concave near the beaks, straight behind ; 

 base broadly and regularly curved; posterior end narrow and 

 prominent. Surface marked by a few lines of growth. 



Figure, magnified about a third. 



y 



