CRETACEOUS FOSSILS. 173 



C. Brewerii, n. s. 



PL 24, Fig. 155. 



Shell moderate in size, subequilateral, subquadrate, a little 

 wider than long ; beaks central, strongly incurved ; hinge-line 

 nearly straight; anterior and basal margins forming a regular 

 curve ; posterior end abruptly truncated. Surface, in advance of 

 a rounded, umbonal ridge, regularly convex ; posterior slightly 

 concave and sloping rapidly towards the posterior margin ; orna- 

 mented by about twenty-five uniform subflattened ribs, with per- 

 fectly flat interspaces; the surface of these ribs is sometimes 

 plain, sometimes faintly grooved longitudinally ; the interspaces 

 are crossed by numerous minute, squamose lines, which occa- 

 sionally are visible on the ribs themselves. 



Figure, natural size. 



Localities: This beautiful shell is abundant in the beds of Division B., east of 

 the north end of Canada de las Uvas ; also found in the same formation around 

 Martinez, although always small at the latter locality. 



C. (Protocardium) Placerensis, u. s 



PL 24, Fig. 156. 



Shell small, subcircular, nearly equilateral; beaks central, 

 moderately prominent, with the posterior end faintly subtruncated 

 obliquely outwards; anterior end regularly convex. Surface 

 marked by minute, irregular, concentric lines, and by seven or 

 eight radiating ribs on the posterior face ; the largest of these 

 is on the umbonal angle, and is bounded by two deep grooves; 

 the others are placed between it and the margin of the shell, and 

 decrease regularly in size, leaving a narrow space between the 

 last one and the margin plain. Internal margin plain. Hinge 

 slender. 



Figure, magnified three times. 



Locality: Texas Flat, Placer County. Collection of the California Academy of 

 Natural Sciences ; collected by Dr. Trask. 



