204 PALAEONTOLOGY OF CALIFORNIA. 



small, distinct, marginal or submarginal. Surface marked by 

 fine, linear, radiating ribs, often diehotomous, sometimes laterally 

 undulated and crossed by concentric lines of growth, which some- 

 times become squamose. Muscular scar large. Under valve 

 unknown. 



Figure, natural size. 



Localities: Division A., Texas Flat; Pence's Ranch; Chico Creek, -&C. 



OSTREA, Linn. 



0. Brewerii, n. s. 



PI. 26, Fig. 191. 



Shell large, thick, elongated, tapering pretty regularly to- 

 wards the beak. Surface squamose, not plicate. Ligament pit 

 subquadrate to triangular, median depression broad and deep, a 

 cross section forming from a fourth to a fifth of a circle ; upper 

 end strongly curved towards the left side. Internal margin 

 plain. Shell gregarious. 



Figure, two-thirds of the natural size. 



Locality: Cow Creek, Shasta County; collected by Mr. Brewer. 



0. MALLEIFORMIS, n. S. 

 PI. 31, Fig. 272. 



Shell auriculate, nearly equivalve, the lower valve a little the 

 most convex, long, narrow, slightly curved to one side ; hinge 

 very small, cardinal margin descending on each side of the hinge 

 at a broad angle, nearly straight; ear on the right side biangular, 

 nearly square ; opposite ear uniting with the side in a broad curve. 

 Surface not squamose, marked by a few lines of growth. Mus- 

 cular scar nearly central longitudinally, small. 



Figure, natural size. 



Localities: Siskiyou Mountains, in California and Oregon; also at Jacksonville, 

 Oregon (Division A.). 



