LAMELLIBRANCHIATA. 461 



reflexed, with a slight constriction behind it and without any conspicuous 

 limiting notch. 



"Valves moderately convex in the posterior part, gibbous in the middle 

 and umbonal portions. 



Beaks sub-anterior, not prominent, slightly incurved. Umbonal slope 

 angular above, extending to the posterior extremity, often merging into the 

 general convexity of the shell below the middle. Post-cardinal slope nar- 

 row, slightly concave. 



Surface marked by fine concentric striae, which are sometimes somewhat 

 fasciculate, producing gentle undulations of the surface. 



Two specimens measure respectively 15 and 20 mm. in length, and 5 and 

 7 mm. in height. 



This species differs from the preceding in its elongate lanceolate form, 

 obtusely angular umbonal ridge and in the pointed posterior extremity. 



Formation and localities. In the shales of the Hamilton group at Fultonham, 

 Schoharie county ; Sherburne and Norwich, Chenango county ; Pratt's falls, 

 Onondaga county; and on the shores of Cayuga and Canandaigua lakes, N. Y. 



Prothyris alata, n. sp. 



PLATE XCIV, FIG. 7. 



Shell small ; body of the shell narrowly ovate, acute ; length twice the height ; 

 basal margin gently curving. Posterior extremity pointed. Cardinal line 

 more than half the length of the shell. Anterior end truncated by a vertical 

 groove, beyond which is a narrow parallel fold, which is notched at the base. 



Valves regularly convex. 



Beaks sub-anterior, low, not defined. Umbonal slope angular, marked 

 below by two or three radiating striae, which extend from the umbo to the 

 posterior extremity of the shell. Cardinal slope alate, marked by distinct 

 radiating striae. 



Surface marked by very fine concentric striae, and the cardinal slope by 

 fine radiating striae. 



The specimen described has a length of 8.5 mm. and a height of 4 mm. 

 59 



