204 PALAEONTOLOGY OF CALIFORNIA. 



small, distinct, marginal or submarginal. Surface marked by 

 fine, linear, radiating ribs, often dichotomous, 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 Kanch; Chico Creek, &c. 



OSTREA, Linn. 

 O. BREWERII, n. s. 



PL 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. 

 PL 81, 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.). 



