LAMELLIBRANCHIATA. 327 



sub-angular, arching upward and extending from the beak to the post-inferior 

 extremity. Post-cardinal slope short, descending abruptly from the angular 

 umbonal ridge to the obliquely truncate posterior margin. 



Test thin, marked by very fine concentric striae which do not appear to 

 have been fasciculate. 



Hinge comparatively short, furnished with more than twenty small teeth, 

 which are continued in a row under the beaks without interruption. Mus- 

 cular scars faintly marked. Clavicular ridge or septum very strong, sharply 

 defined and curved. 



Three specimens of average size measure respectively 17, 16 and 12 mm. 

 in length, and 13, 13 and 10 mm. in height, and 11, 10 and 8 mm. in the 

 depth of both valves. 



This species is somewhat similar in form to Nucula varicosa, but it attains a 

 much larger size, the umbonal slope is distinctly angular, and the surface is 

 usually smooth, or marked by very fine concentric lines, while the impression 

 of the curved muscular ridge or clavicle just anterior to the beaks serves to 

 distinguish the casts, in which condition the specimens are usually found. 



This form presents a wide variation in size and specific characters. Some of 

 the specimens here included under this species were originally designated as 

 N. Nyssa, but larger collections have shown that they are only extreme varieties 

 of form. That species is now limited to the original specimens from the shales 

 in the western portion of the State, which seem to possess sufficient differences 

 to warrant a separate specific designation. 



Formations and localities. In the shales of the Hamilton group, in Onondaga 

 county ; on the shores of Otisco, Skaneateles, Cayuga and Seneca lakes, N. Y. ; 

 and at Patterson's creek, Va. Several small specimens, apparently of this spe- 

 cies, have been found in the Marcellus shale, at Leroy and West Bloomfield, 

 N. Y. 



