CRETACEOUS FOSSILS. 195 



Localities: "Rag Canon," Napa County; and San Luis Gonzaga Ranch, Pa- 

 checo's Pass (Division A.). 



All of the specimens are more or less rolled, so as to obliterate any very line 

 markings. 



A. DECURTATA, n. S. 



PL 31, Fig. 265, and 265 a. 



Shell thick, convex, triangular; beaks high, very approxi- 

 mate, subcentral ; base broadly and regularly convex, most promi- 

 nent in the middle; anterior end rounded below ; posterior end 

 straight, oblique and truncated behind the umbonal ridge, at an 

 acute angle with the rest of the surface. Hinge very short; 

 area short and broad. Surface marked only by indistinct lines of 

 growth. 



6 



Figures, natural size. 



Locality: A single specimen, found at Rag Canon with the preceding. 



CUCULL^A, Lam. 

 C. Matiiewsoxii, n. s. 



PL 31, Fig. 266. 



Shell large, thin, gibbous, subquadrate, rounded in front and 

 on the base, and truncated behind; beaks large, prominent, sub- 

 central, incurved, and somewhat remote ; area long, narrow ; 

 hinge-line nearly as long as the greatest length of the shell. Sur- 

 face marked by a large number of small, regular ribs, rounded 

 on the surface, sometimes grooved longitudinally and with acute 

 interspaces ; posterior to the umbonal angle these ribs are much 

 smaller than on the rest of the surface; the whole surface is 

 crossed by fine lines of growth. Inner margin of the valves 

 crenulated ; inner plate well marked, but not very elevated. 



Figure, natural size. 



Localities: Martinez, collected by Mr. Mathewson ; and a single specimen from 

 Clayton, below the coal-veins. 



