LAMELLIBRANCHIA TA. 
101 
mm. A right valve has a length of 46 mm., height 40 mm., hinge-line 
48 mm. 
This species differs from P. Chemungensis in being shorter, proportionally more 
extended on the hinge-line, with wider ligamental 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, Chemung county, and Smithboro, Tioga county, N. Y. 
Pterinea rigida. 
PLATE, XVI, FIGS. 5, 6. 
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, depressed-convex below. Bight 
valve unknown. 
Hinge-line straight; length about equal to the 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 fiat 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- 
