146 
PALAEONTOLOGY OF NEW YORK. 
basal margin is broadly rounded; posterior extremity recurved at a little less 
than a right angle. 
Left valve more convex than the right and gibbous above, obscurely 
angular along the post-cardinal slope. This difference in the valves is also 
shown in the direction of the striae of growth. 
Hinge-line straight, about three-fourths as long as the length of the shell. 
Beaks near the anterior fourth of the hinge, prominent in the left valve, 
acute, arching over the cardinal line; in the right valve depressed, rising only 
as high as the hinge. Umbonal region of the left valve moderately gibbous, 
subtending an acute angle. 
Anterior end short, small, limited by an oblique byssal depression which 
extends along the base about one-third the length of the shell; extremity 
acute. Wing large, extending along the shell to near the posterior extremity ; 
margin obliquely truncate, concave just below the hinge-line, which is 
abruptly produced. 
Test thin, marked by fine, interrupted or undulated radii, which are more 
strongly marked on the body and wing, and obsolete on the anterior of the 
valve ; the surface is also marked by fine concentric striae, which are crowded 
into fascicles at unequal distances, undulating the surface ; they turn abruptly 
outward, just below the hinge, and are lamellose on the anterior part of the 
valve. 
Interior unknown. 
A specimen preserving both valves has a length of 45 mm., height 28 mm., 
hinge-line 35 mm. The right valve is somewhat smaller. Another 
specimen has a length of 42 mm., height 28 mm., hinge-line 36 mm. 
This species resembles in form P. Sao, but is larger, the wing furrow more 
strongly defined on the left valve, the concentric lines more acutely recurved 
at the basal angle, and the radii are finer and more numerous. 
Formation and locality. In the Upper Chemung group, the middle beds of 
the series, as seen at Warren, Pa. 
