544 NORTH AMERICAN INDEX FOSSILS. 



Very abundant in the Bear River of Wyoming, Utah and Idaho. 



508. C. occidentalis Meek and Hayden. Cretacic. 

 Shell thick, subtrigonal. Average length and height 1 inch ; 



convexity . 75 inch. Slopes from beak subequal ; the anterior 

 slightly concave, the posterior somewhat convex. Base broadly 

 curved. Beaks elevated, gibbous. 



Especially abundant in the brackish water strata of the Mon- 

 tanan of Wyoming, Montana, Assiniboia, Alberta ; also present in 

 the Laramie throughout this region. 



CXLIII. Sph^rium Scopoli. 

 Small, thin, inflated, rounded. Sculpture concentric, never 

 strong. Cardinal teeth usually two in each valve, variable, thin, 

 often nearly parallel to the hinge line or in part defective. Lateral 

 teeth compressed, lamelliform, the anterior shortest. Ligament 

 short and feeble, deep-set in a groove. Muscle impressions near 

 margin and scarcely apparent. Pallial line simple. Fresh water. 

 Cretacic-Recent. 



509. S. planum Meek and Hayden. (Fig. 744, g t h.) Cretacic. 

 Much compressed, with very small nearly central beaks that 



scarcely rise above the hinge line. Surface marked by obscure 

 and irregular concentric striae. 



Judith River (Pierre) : Nebraska, Montana, Assiniboia. Laramie : 

 Wyoming. 



510. S. formosum Meek and Hayden. (Fig. 744, d-f.) 



Cretacic-Lower Eocenic. 



Very small (length less than .2 inch). Beaks somewhat tumid 

 and rising a little above the hinge line, a little anterior of the 

 middle. Surface marked by distinct and regular concentric lines. 



Pierre (Belly River) of Alberta and Assiniboia. Fort Union of 

 Montana. 



CXLIV. Venericardia Lamarck. 



Shell rounded or cordate, inequilateral, with radiating ribs. 

 Umbos prominent. Basal margin crenulated internally. Liga- 

 ment external. Hinge thick with no lateral teeth but with the 

 posterior cardinal much prolonged. Adductor impressions un- 

 equal. Pallial line simple. Cretacic-Recent. 



511. V. smithi Aldrich. (Fig. 747, b.) Eocenic. 



