206 PALAEONTOLOGY OF CALIFORNIA. 



Upper valve broad, suboval, very high ; end3 rounded, sloping, 

 nearly straight above, base nearly straight, except in the median 

 emargination ; marked by from twenty-five to thirty radiating, 

 subangular ribs, with nearly equal interspaces; from eight to 

 ten of these ribs on the middle of the shell are a little shorter 

 than the others, producing a straight, shallow emargination on 

 the basal edge of the valve. Internal margin of the base deeply 

 notched; hinge teeth of the upper valve robust; median plate 

 rather long and prominent (in a young specimen); loop un- 

 known. 



A young upper valve, .6 of an inch wide, is straighter above 

 and semicircular on the sides and base; it has but seventeen ribs, 

 two or three of which are dichotomous, none so angular as in the 

 adult shell, and there is no emargination of the base. 



Adult shells vary very much in convexity and costation, one 

 specimen having eight and another ten ribs in the median por- 

 tion. But one fragmentary beak has been seen, as represented 

 in the figure. 



Figures, natural size, except 194 b, which is from a smaller specimen and is 

 slightly magnified. 

 Localities: Division A., Texas Flat, Placer County. 



