288 Thirty-fifth Keport ox the State Museum. 



Formations and localities. This species has been recognized in a 

 single ill-preserved specimen from the Corniferous limestone, at 

 Stafford, Genesee county, N. Y. ; casts of the interior, and a 

 single extravagant form preserving the surface characters, have 

 been obtained from limestone of the same age near Columbus, 

 Ohio. It is abundant in the Hamilton group at numerous localities 

 from Schoharie to Cayuga lake, more rarely in the western part of 

 the State; though occurring at Hamburg and elsewhere on Lake 

 Erie shore. It is found in the upper beds of the Chemung group in 

 Steuben county, N. Y., and Tioga county, Pa. 



PTERIXEA ItfTERSTRIALIS. 



Pterinea inter strialis, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell large ; left valve obliquely ovate, narrow above ; length and 

 Jieight nearly equal ; anterior and basal margins regularly curved ; 

 posterior margin produced. Left valve moderately convex, de- 

 pressed toward base. Eight valve unknown. Hinge-line straight, 

 about equal to the length of the shell. Beak acute, inclined for- 

 ward. Umbonal region not prominent, narrow, limited by the 

 cardinal expansions ; subtending an acute angle. Wing triangular, 

 large ; margin concave ; extremity acute. Ear triangular, small. 

 Surface marked by about twenty regular, strong, rounded rays which 

 originate on the upper part of the umbo and continue undivided to 

 the margin ; with broad, flat interspaces marked by from two to four 

 slender, sharp radii, and crossed by fine concentric striae. About 

 twelve of the strong rays reach the margin of the valve below the 

 middle of its height, the remainder come out on the upper anterior 

 margin. The wing shows the radii somewhat less strong than the 

 body of the valve. The ear is marked by strong radii. 



The specimen described has a greatest length of about 55 mm., 

 height nearly 58 mm., hinge-line, from beak to extremity of wing, 

 about 58 mm. 



This species somewhat resembles P. flabella ; and differs in its less 

 convexity, more numerous strong rays with finer intermediate lines, 

 and less conspicuous concentric striae. 



Formation and locality. In sandstone of the Chemung group at 

 Chemung Narrows, Chemung county, N. Y. 



