70 [ Senate 



of R. wilsoni, but is a larger and relatively broader shell, the mesial ele- 

 Tation is more distinct, and the general form is less rotund. It is possible 

 that more extensive collections may prove this species to be an extreme 

 variety of the preceding. 



Geological position and locality. Shaly limestone of the Lower Hel- 

 derberg group, Albany and Schoharie counties. 



RhYNCHONELLA PYEAinDATA. 



Pal. N.Y. Vol.iii, pi. 32, f. 1 & 2. 



Shell pyramidal, subpentagonal ; outline subtriangular : ventral 

 valve nearly flat or depressed-convex, more or less abruptly 

 deflected at the margins towards the other valve ; beak promi- 

 nent, flattened, and closely incurved over that of the opposite 

 valve : dorsal valve very gibbous, declining from near the front 

 towards the beak ; beak angular, incurved, flattened : lateral 

 slopes with a distinctly impressed suboval space beneath the 

 beaks, which is bounded by an angular fold extending from the 

 beak downwards to the valves at the first strong plication. 

 Surface marked by from thirteen to twenty-two simple strong 

 subangular plications, four to six of which are more or less 

 elevated towards the front of the dorsal valve into a mesial pro- 

 minence, and three to five depressed on the ventral valve so as 

 to form a shallow sinus, and produced iif front into a distinct 

 linguiform projection. 



The plications on the front of this shell have but very faint traces of 

 the longitudinal depressed lines so common in this type of Rhynchonella, 

 though occasionally remains of very fine closely arranged zigzag striae are 

 seen crossing them near the margins of the valves. 



In some of its varieties, this species approaches tbe R. nucleolata ( pi. 

 31, f. 1 & 2), but differs conspicuously in being usually larger, and in its 

 more angular outline and much stronger plications. The beaks are also more 

 prominent in this species, and the dorsal valve is more extremely elevated 

 near the front : there arc also differences in the visceral impressions. 



