30 PALAEONTOLOGY OF CALIFOENIA. 



by an obtuse angle; basal margin regularly rounded; anterior 

 sinuous. Surface convex below, excavated towards tbe posterior 

 wing ; marked only by lines of growth. Anterior wing unknown. 



Figure, natural size. 



Locality: Star District, Humboldt County, Nevada Territory. Collected by 

 Mr. Homfray. 



A. MUCRONATA, 11. S. 

 PL 5, Fig. 27. 



Shell small, compressed, oblique ; beaks central ; anterior 

 basal margin rounded, obliquely truncated ; posterior prominent ; 

 anterior ear unknown ; posterior broad at tbe base, emarginate 

 below, and produced at tbe extremity in the form of a slender 

 spine-like process. Surface marked by fine, irregular, linear, 

 radiating ribs, crossed by a few lines of growth. 



Figure, natural size. 



Locality: Gifford's Ranch, Plumas County. 



HALOBIA, Bronn. 



? II. dubia, n. s. 



PL 5, Fig. 28, a, b. 



Shell equivalve, slightly oblique, very much compressed. Beak 

 small, hardly projecting beyond the hinge margin; nearly (if 

 not exactly) central. Cardinal margin straight. Surface marked 

 by irregular, straight, or waved radiating ribs. These ribs are 

 marked by fine impressed lines, one of which occasionally be- 

 comes deeper than the others, thereby causing a bifurcation. 

 The spaces between the ribs are deep, and rarely more than half 

 as wide as the ribs themselves. In most specimens the surface is 

 crossed by oblique concentric undulations, as in some species of 

 Lcptcena. 



The close resemblance of this shell to the genus Orthis is sufficient to throw 

 some doubt on its correct generic determination. I have examined a large suite 

 of specimens, and have not been able to detect any trace of an area: this fact, 



