LAMELLIBRANCHIATA. 1 39 



more elevated beak, narrower anterior end, more abrupl 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 A.LATA. 



1'L.ATE XXIII, FIGS. 25, .v, [21, 22?); vxn PLATE I. XXXV. PIG. 28 



Ptychopteria alata, Hall. Pal. N.Y., vol. v. i>t. 1. Plates and Explanations: PI. •_':!. figs. 25, 26 (21, 



22 1). 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, ami 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 stria- 

 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 nun., height 2<i mm., and hjnge-line 

 35 mm. 



