CRETACEOUS FOSSILS. 193 



by a few faint lines of growth. Posterior end not thickened, as 

 in the preceding species. 



Figure, natural size. 



Localities: Division A., near Martinez; also at Chico Creek and Pence's Ranch. 

 Butte County. 



This species is very distinct from the preceding. The difference in outline is 

 very marked; it is very much compressed; the shell is thin; the posterior thicken- 

 ing, so characteristic in the former, is entirely absent in this ; and the internal 

 plate in front of the beak is thinner, and is inclined more strongly forward. It 

 is rare, five specimens only having been found. 



ARCA, Lam. 

 A. Breweriana, n. s. 



PI. 25, Fig. 181. 



Shell small, longer than wide; beaks prominent, incurved, 

 approximate, and inclined forward, placed in advance of the 

 middle; umbones broad ; anterior end rounded and sloping in- 

 wards; posterior end obliquely truncated and uniting with the 

 base by an abrupt angle; basal margin slightly rounded in 

 front, straight behind, and parallel with the hinge or sloping 

 upwards; umbonal angle sharp, with the surface in front of it 

 very convex, behind it somewhat excavated; area narrow, al- 

 most as long as the shell ; marked by a few small, closely placed 

 angular lines. Surface marked by numerous small, variable ribs, 

 smallest behind the umbonal ridge ; four or live of these ribs on 

 the anterior end of the shell are larger than the others, placed 

 wider apart, and are sometimes slightly nodose; the others are 

 occasionally obsolete ; these ribs are crossed by numerous very 

 irregular lines of growth. Hinge slender; the teeth below the 

 beaks, to the number of two or three only, are transverse; those 

 adjoining arc oblique, the angle rapidly growing wider, and those 

 at the extremities are all horizontal; this is most strongly 

 pal. vol. i — 25 



