LAMELLIBRANCHIATA. 101 



mm. A right valve has a length of 46 mm., height 40 mm., hinge-line 

 48 mm. 



This species difiers from P. Chemungensis in being shorter, proportionally more 

 extended on the hinge-line, with wider ligamental area, more distinct radii, 

 and greatly difterent proportions of length and breadth, as shown in the right 

 valves. 



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

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



Pterinea rigiua. 



PLATE, XVI, FIGS. 5, G. 

 Pterinea rig'ida. Ham,. Pal. N. Y., vol. v, pt. 1. Plates ami Explanatioiis : PI. Hi, tifjs. 5, tj. Jan., 1S83. 



Shell of medium size, su))-rhomboidal, erect; body ovate, narrow al)ove ; 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, depressed-convex below. Right 

 valve unknown. 



Hinge-line straight ; length about equal to tlie length of the valve. 



Beak acute, prominent, directed forward, rising to the hinge-margin. 

 Umbonal region 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. 



Interior unknown. 



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



