LAMELLIBRANCHIATA. 369 



undulations are more strongly marked. It differs from G. circularis in the 

 absence of any mesial cincture. 



Formation and localities. In the shales of the Hamilton group, on the shores 

 of Canandaigua and Seneca lakes, N. Y. 



Grammysia glabra, n. sp. 



I'LATE XCIII, FIG. 24. 



Shell of medium size, ovate ; length one-third greater than the height ; basal 

 margin regularly convex, with a slight constriction anterior to the middle of 

 its length. Posterior extremity narrowly rounded. Cardinal line arcuate, 

 declining posteriorly. Escutcheon narrow. Anterior end large, declining 

 rapidly from the beaks, and regularly rounded below the lunule. 



Valves regidarly convex in the lower and posterior portions, becoming 

 gibbous in the middle and above. 



Beaks at about the anterior third, large, much elevated above the hinge- 

 line. Umbonal region regularly convex. Umbonal slope rounded, not 

 defined. There is a slight depression extending almost vertically from the 

 umbo, and producing a gentle constriction in the basal margin. 



Surface marked by fine, sub-equal, concentric striae, which become fascicu- 

 late, and often rise into low undulations on the anterior portion of the shell. 

 Anterior muscular impression large, situated below the limits of the lunule. 

 Pallial line, in the cast, marked by a row of inconspicuous nodes, following 

 near to the ventral margin. Posterior scar large and superficial, situated on 

 the post-cardinal slope. 



Two specimens measure respectively 53 and 57 mm. in length, and 36 and 

 38 mm. in height. 



This species most nearly resembles G. obsoleta, but in its natural condition it 

 is not so broadly ovate, and it has no indication of the fold and sinus on the 

 anterior end, as in that species. 



Formation and localities. In the upper sandstones of the Chemung group, at 

 Warren, Pa., and doubtfully at Clark's farm, near Panama, N. Y. 



