CRETACEOUS FOSSILS. 



199 



it to be the young of P. Traskll, but have now three specimens varying only in 

 size one or two-hundredths of an inch, and consequently cannot resist the belief 

 that it is a distinct species. 



P. COMPLEXICOSTA, n. 8. 

 PI. 33, Fig. 97, 97 a. 



Shell moderate in size, thin, equivalve, nearly equilateral ; 

 sides and base forming a regular curve, slightly elongated ; the 

 right side of the upper valve, and corresponding side of the 

 lower, a little the most convex above ; upper valve with the left 

 ear a little concave on its lateral margin ; shape of the right ear 

 unknown ; right ear of lower valve narrow, produced, and deeply 

 emarginate. Surface marked by about twelve or fourteen radi- 

 ating ribs, with sometimes an equal number of smaller ones 

 intercalated ; these are more strongly marked on the cast than 

 externally ; besides the ribs, the entire surface is closely sculptured 

 by minute radiating lines, very variable in size. 



Figure, natural size, and a magnified view of the surface. 



Common in the white limestone of the Shasta Group in Morgan Valley, south 

 of Clear Lake, collected by Professor Whitney. Although .the species is abun- 

 dantly represented at this locality, no entire specimens have ever been obtained, 

 owing to the character of the matrix. By the study of a large number of fragments, 

 I have been enabled to arrive at all the important specific characters, except the 

 shape of the ear, which is represented broken in the figure, as it occurs in the best 

 specimen. Two or three casts before me give the outline, and the details of the 

 surface are drawn from impressions in the matrix. 



P. INTERRADIATUS, 11. S. 

 PI. 33, Fig. 98, 98 a. 



Shell small, subcircular, equivalve, equilateral, compressed, 

 thin; upper valve, ears equal, moderately large; lower valve, right 

 ear long, deeply and narrowly emarginate. Surface marked by 

 very numerous fine radiating lines, and obscure lines of growth. 

 Internal surface of both valves bearing eight straight, equidis- 

 tant, large ribs, of variable length among themselves, extendi] \g 



