126 [ Senati 



of the hinge ; beak incurved : ventral valve convex, having 



two prominent ridges extending at an angle of about 90o from 



the beak to the anterior and lateral margins : the space between 



these ridges is occupied by a very broad, deep, sloping mesial 



sinus extending from the beak to the front, where it terminates 



in a broad triangular extension ; beak closely incurved. Surface 



smooth, or marked by faint lines of growth. 



This remarkable shell differs so widely from any other species hitherto 



found in our rocks, as to be at once recognized merely by its form. I have 



placed it provisionally in the Genus Rhynchonella, not having yet had an 



opportunity of seeing its internal characters. 



Rhynchonella inequiplicata ( n. s.). 



Pal. N.Y. Vol. iv. 



Shell subquadrilateral, broader than long ; hinge and area un- 

 known, apparently rounded at the lateral extremities : dorsal 

 valve the larger, convex, rising in the middle into an undeiined 

 mesial fold ,• entire surface marked by about twelve or thirteen 

 irregular bifurcating plications, three or four of which occupy 

 the mesial elevation ; beak distinctly incurved : ventral valve 

 most prominent near the umbo, depressed in the middle into a 

 shallow undefined sinus, which is occupied by three or four 

 smaller plications, and terminates in a short subangular projec- 

 tion. Finer surface markings unknown. 



Of this rare species, I have yet only seen a single and somewhat mu- 

 tilated specimen. It appears to be quite distinct from all the other forms 

 known in our rocks. 



Geological position and locality. Limestone of the age of the Upper 

 Helderberg group, Western New- York : particular locality unknown. 



