No. 109.] 165 



mesial sinus ; plications on each side very oblique ; area equal- 

 ling or exceeding the ventral valve, receding from the hinge- 

 line towards the front of the shell, and thus narrowing the 

 exterior area of the ventral valve. 



"This is a well marked species, the most conspicuous feature of which 

 is the very narrow ventral valve, whicli, in the specimen figured is less 

 than the area in width ; while the dorsal valve is much wider in the mid- 

 dle, and curves somewhat gradually towards the extremities. 



Geological position and locality. In shales of the Hamilton group : 

 Covington, Genesee county. 



Spirifkr axgusta. 



Cyrtia biplicata (n.s.). 



Pal. N.Y. Yol.iv. 



Shell triangular pyramidal, about twice as broad as long ; 

 hinge not quite equalling the greatest width of the shell, sub- 

 angular or a little rounded at the extremities : dorsal valve 

 unknown : ventral valve extremely prominent at the beak, 

 from which it declines abruptly to the sides and front, having 

 a moderate sinus extending to the apex of the beak, and bounded 

 on each side by a single ridge or plication ; beak attenuate and 

 pointed, not arched, removed from the hinge by the intervening 

 high, slightly arcuate, triangular area ; foramen very narrow 

 and closed. Surface apparently smooth. 



The specimen examined is not entire ; but so far as can be determined, 

 it presents the remarkable characteristic of a single sharply angular pli- 

 cation upon each side of the sinus. If other plications existed, they pro- 

 bably extended but little beyond the margin of the shell. 



Geological position and localilij. In limestones of the Upper Heldcr- 

 berg group, Erie county, N.Y. 



