394 NORTH AMERICAN INDEX FOSSILS. 



58. C. logani Salter. (Fig. 502, a-b.) Ordovicic. 

 Beaks large. Muscle scars faint ; hinge nearly straight. 

 Black River : Wisconsin, Ontario. 



59. C. socialis Ulrich. (Fig. 502, e-f.) Ordovicic. 

 Shell very small, quite inequilateral. Convexity moderate. 

 Very abundant in the Black River shale of Minnesota. 



60. C. alta Hall. (Fig. 502, k-l.) Ordovicic. 

 Erect, subtriangular. Hinge plate as in C. obliqua. 



Galena : Illinois, Wisconsin, Minnesota. 



61. C. levata Hall. Ordovicic. 

 Differs from C. albertina in its greater proportional height and 



convexity, and in the absence of a pit beneath the beak. 



Trenton : New York, New Jersey, Wisconsin. Lorraine : 

 Pennsylvania. 



62. C. astartiformis Salter. Ordovicic. 

 Differs from C. obliqua in its larger size (about twice that of 



the smaller form), in its smaller and more bent beaks, and in its 

 stronger and more irregular concentric striae. 



Trenton : New York ?, Quebec, Lake Winnipeg, Manitoba. 



63. C. fecunda Hall. (Fig. 502, g-h.) Ordovicic. 

 Hinge plate arcuate. • Muscle scars faint. Form suboval. 

 Maquoketa : Illinois, Iowa, Wisconsin, Minnesota. 



64. C. calvini Ulrich. (Fig. 502, «.) Ordovicic. 

 Differs from C. fecunda in its larger size and greater posterior 



height. 



Maquoketa : Illinois, Iowa. 



65. C. obliqua Hall. (Fig. 502, i-j.) Ordovicic. 

 Very small. Concentric lines comparatively strong. Hinge 



plate bent at nearly a right angle. 



Cincinnati group of Ohio, Indiana. Maquoketa of Iowa and 

 the Northwest. 



66. C. albertina Ulrich. (Fig. 502, m.) Ordovicic. 

 Hinge plate sharply bent ; a small pit present beneath the beak. 

 Upper beds of Cincinnati group of Ohio and Minnesota. 



67. C. machaeriformis Hall. Siluric. 

 Shell thin, elongate. Beak subanterior. Surface marked with 



minute, equal, concentric striae. Dimensions i|x | inches. 

 Clinton : New York. 



