134 
PALAEONTOLOGY OF NEW YORK. 
margin broadly curved; posterior extremity abruptly or sub-angularly 
recurved. 
Left valve convex, gibbous on the umbo and obtusely sub-angular along 
the post-cardinal slope. Right valve less convex, somewhat smaller than the 
left. 
Hinge-line straight; length about two-thirds the length of the valve. 
Beak near the anterior fourth of the length of the hinge, directed forward; 
beak of left valve prominent, acute, incurved over the hinge-line; beak of 
right valve obtuse, and not elevated. Umbonal region of the left valve 
gibbous and angular, of the right valve convex, subtending a very acute 
angle. 
Anterior end small; margin rounded; extremity acute. Byssal depression 
oblique, directed backward. Wing narrow, triangular, joining the body at the 
posterior end; margin obliquely truncate, scarcely concave; extremity 
apparently not produced. 
Surface marked by fine radiating strise, which are more conspicuous, on the 
body of the valve and upon the wing, and are very obscure on the anterior 
end: these are crossed by fine concentric striae of growth which, at irregular 
intervals, a^e raised into fascicles, usually more conspicuous on the umbo 
and post-cardinal slope. On the right valve the radii are usually obsolete. 
Interior unknown, except the usual linear groove along the hinge-line. 
A large left valve has a length of 40 mm., height 20 mm., and hinge-line 
28 mm. Another is 38 mm. in length, height 20 mm., and hinge-line 27 mm. 
In this species the body is more oblique than in P. Salamanca; the angularity 
of the left valve is in a direct line down the middle of the body; the shell 
is comparatively narrower; the extremity of the wing is not produced; the 
byssal depression is more oblique. The want of striae on the anterior end and 
their obsolescence on the right valve are marked characters. It is proportionally 
much longer than P. Sao, with which it is associated. 
Formation and locality. In the lower part of a conglomerate of the Chemung 
group at Panama, Chautauqua county, N. Y. 
