292 Thirty-fifth Report on the State Museum. 



area, more distinct radii, and greatly different proportions of length 

 and breadth, as shown in the right valves. 



Formation and localities. In the Chemung group at Buck's quarry 

 and Chemung, Ghemung county, and Smithboro, Tioga county, N. Y. 



Pterin a rigida. 



Pterinea rigida, Hall. Pal. N.Y., vol. v. pt. 1. Plates and Explanations: PI. 

 16, figs. 5, 6. Jan. 1883. 



Shell of medium size, sub-rhomboidal, erect ; body ovate, narrow 

 above ; length somewhat less than the height ; margins regularly 

 rounded, slightly extended on the post-basal portion and straight 

 along the posterior side. Left valve moderately convex above, de- 

 pressed-convex below. Right valve unknown. Hinge-line straight ; 

 length about equal to the length of the valve. Beak acute, promi- 

 nent, directed forward, rising to the hinge-margin. Umbonal re- 

 gion elevated, subtending an acute angle. Ear small, limited by 

 the moderate byssal sinus and broad sulcus. Wing large, broad- 

 triangular, undefined ; margin slightly concave ; extremity angular. 



Surface marked with about thirty slender, rigid, rounded rays ? 

 which are sometimes flat or grooved along the summit, continuous 

 from the upper part of the umbo ; with wider flat interspaces 

 which sometimes show a fine intermediate ray coming in below the 

 umbo. The cardinal expansions are marked by rays of the same 

 character. Lines of growth cross the surface. 



A medium sized specimen has a length of 31 mm., height 34 mm., 

 hinge-line about 35 mm. A smaller specimen is 26 mm. in length, 

 25 mm. in height, hinge-line 24 mm. 



This species bears a general resemblance to P. Chemung ensis ; it 

 differs in its continuous and comparatively stronger rigid radii. The 

 group of four species, beginning with P. Chemung 'ensis , is well 

 marked and have many characters in common; but nevertheless, 

 present differences which warrant their specific separation. 



Formation and localities. In the Chemung group, at several locali- 

 ties in Chemung county, N. Y. 



Pterinea prora. 



Pterinea prora, Hall. Pal.N.Y., vol. v, pt. 1. Plates and Explanations: PL 

 16, figs: 4,13,14. Jan., 1883. 



Shell of medium size or larger, sub-rhomboidal, oblique ; body ovate, 

 narrow above; length one-fourth greater than the height ; margins 



