TERTIARY FOSSILS. 31 



ARCA, Lam. 

 A. SULCICOSTA, n. s. 



PI. 9, Fig. 53, 53 a. 



SHELL thin, broad; beaks prominent, incurved, approximate, 

 slightly twisted anteriorly; hinge line short; ends and base pretty 

 regularly rounded, posterior basal portion a little the most promi 

 nent; area very narrow, slightly sunken. Surface marked by 

 about twenty-five prominent square ribs, with flat, equal inter 

 spaces ; these ribs are each marked by a more or less distinct 

 median groove, and crossed by pretty strong concentric lines of 

 growth, breaking up the surface into a beautiful beading. Hinge 

 straight, composed of numerous fine teeth, very small and irre 

 gular in the middle, larger and slightly oblique towards the ends. 



Figure 53, natural size; fig. 53 a, a magnified view of a few ribs on the ante 

 rior portion, to show the details of sculpture. 



Locality and position: Found with Dosinia Staleyi, on Mark West Creek, Sono 

 ma County, by Mr. V. S. Staley. 



This species diflers from A. canalis, Con., in being less ventricose, less promi 

 nent posteriorly, more produced in front, deeper from beaks to base, in being 

 much thinner, and in the beading of the ribs. 



YOLDIA, Holler. 

 Y. COOPERII. 



PI. 9, Fig. 54. 

 (Y. Cooperii, Gabb; Proc. Cal. Academy, 1865, p. 189.) 



SHELL thin, subcompressed, very inequilateral; beaks minutely 

 placed in advance of the middle, becoming more anterior as the 

 shell increases in size; anterior end narrow, subacuminate ; pos 

 terior end broadly rounded; base most prominent behind the 

 middle, curving upwards to the anterior end. Surface sculptured 



