LAMELLIBRANCHIATA. 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 XXni, FIGS. 25, 2i; (21, >2-l ; and PLATE LXXXV, FIG. 26. 



Ptychopteria alata. Hall. Pal. N. Y., veil. v. pt. 1. Plates and Explaiiaticnis : PI. 23. tig-.-<. 2."), 2(i (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 ol)lique, 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 thi' 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 tine 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 

 (lou))t 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. 



