TERTIARY FOSSILS. 33 



ears unequal; sinus in the right ear of the lower valve mode- 

 rately deep; surface of ears covered by small radiating ribs and 

 fine imbricating lines of growth; surface of valves ornamented 

 by about eight broad, flat, subuodose ribs, arranged in pairs, and 

 a few small linear ribs on the side; besides the ribs, the whole 

 surface is covered by small radiating lines, and crossed by very 

 irregular lines of growth, and very fine imbricating concentric 

 lines. 



Figure, natural size. 

 Locality : With the preceding. 



OSTREA, Linn. 



O. Bourgeoisie 



PI. 11, Fig. 57, 57 a. 

 (0. Bourgeoisii, Re"mond; Proc. Gal. Acad., 1863, p. 13.) 



Shell thin, irregularly oval, compressed and contorted near 

 the beaks. Surface marked by a few irregularly squamose lines 

 and faint radiating ridges. Hinge long oblique; ligament pit 

 deep ; muscular scar large, not very strongly marked. 



Figures, natural size. 



Locality and position: Near Kirker's Pass, Contra Costa County; from the 

 Pliocene. 



O. Atwoodii, n. s. 



PI. 10, Fig. 58, 58 a, and PI. 11, Fig. 58 6. 



Shell broad, irregular, thin ; partly attached, sometimes by 

 nearly the whole of the lower surface ; free surface of lower 

 valve marked by numerous irregular radiating ribs crossed by 

 lines of growth ; upper valves more squamose and not radiated 

 (in the only specimen I have seen). Hinge broad at the base, 



PAL. YOL. II. — 6 



