TERTIARY FOSSILS. 31 



A RCA, 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 differs 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, Moller. 

 Y. Coopemi. 



PI. 9, Fig. 54. 



{Y. Cooperii, Gabb; Proc. Cal. Academy, 18G5, 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 



