CRETACEOUS FOSSILS. 201 



rapidly converging and straight. Right valve, anterior auricle 

 long, truncated at the end, deeply excavated below; posterior 

 auricle broad and obliquely truncated. Surface marked by nu- 

 merous, radiating, squamose ribs, with sometimes smaller inter- 

 mediate ones; the interspaces are marked by oblique lines, 

 producing, under a glass, a woven appearance. These lines are 

 represented too numerously in the figure 187 a. 



Figure 187, natural size. 



Locality : Texas Flat, Placer County (Division A.) ; collection of the California 

 Academy of Natural Sciences ; presented by Dr. Trask, after whom the species is 

 named. 



The species seems to be rare, as there are remains of but two valves in the col- 

 lection. One is very young; the other consists of the internal cast and its cor- 

 responding mould, the substance of the shell having decomposed. The drawing 

 is from a wax cast of this mould. 



P. OPERCULIFORMIS, 11. S. 

 PI. 26, Fig. 188. 



Shell very much compressed, lenticular; basal half of the 

 outline elongated, subsemicircular above; sides converging, 

 straight. Left valve, auricles equal, outer margins converging, 

 upper edges inclined upwards from the beak. Surface polished, 

 showing under a glass minute, equal, concentric ribs. Right 

 valve unknown. 



Figure, natural size. 



Localities: Cottonwood and Hiding Creeks, Shasta County; and Curry's, south 

 of Mount Diablo (Division A.). 



This shell is allied to P. orbicularis, Sow. (P. laminosus, Mant.) ; but is propor- 

 tionally longer, the cars are equal and narrower, and the concentric lamella) are 

 very much smaller, being invisible to the naked eye. 



P. Californicus, 11. s. 



PI. 31, Fig. 270. 



Shell minute, subcircular, nearly equilateral, inequivalve; 

 lower valve somewhat convex, upper valve nearly flat; sides and 



PAL. VOL. I. — 2G 



