LAMELLIBRANCHIA TA. 
139 
more elevated beak, narrower anterior end, more abrupt recurving of the poste¬ 
rior margin, and conspicuous byssal sinus. 
Formation and locality . In a conglomerate of the Chemung group at Panama, 
Chautauqua county, N. Y. 
Ptychopteria alata. 
PLATE XXIII, FIGS. 25, 26 (21, 22?) ; AND PLATE LXXXV, FIG. 26. 
Ptychopteria alata, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 23, figs. 25, 26 (21, 
22 ?).. Jan., 1883. 
Shell large, broadly rhomboidal; body sub-cylindrical, oblique at angle of about 
45° with the hinge-line; height less than two-thirds the greatest length; 
anterior margin oblique, sloping into the broad curvature of the base; 
posterior margin recurved almost rectangularly, slightly acute. 
Left valve gibbous and angular along the axis of the body from the umbo 
to the post-basal extremity. Right valve less convex, and less angular in 
character than the left. 
Hinge-line straight, about one-sixth less than the greatest length of the 
valve. 
Beak very prominent in the left valve, directed forward; more depressed 
in the right valve; situated at about the anterior fifth of the cardinal line. 
Anterior extremity in the right valve small, triangular, acute. Wing 
large, triangular, joining the body at the posterior extremity; the limiting 
furrow is obscure in the left valve, but very marked in the right; margin of 
wing gently concave, nearly vertical; extremity angular, little produced. 
Surface marked by fine radii which (in the specimens seen) are obscure in 
the left valve and nearly obsolete in the right; and by concentric striae 
which are crowded into imbricating ridges. 
Some small right valves found in the same association, and referred with 
doubt to this species, preserve the radii in a much stronger degree. 
Interior unknown. 
A large right valve has a length of 42 mm., height 26 mm., and hinge-line 
35 mm. 
