Lamellibranchiata. 321 



on the umbo, and angular from the umbo to the post-basal extremity. 

 Eight valve unknown. Hinge-line straight, length greater than 

 the height of the valve. Beak in front of the anterior third of the 

 hinge, obtuse, prominent and incurved. Umbonal region gibbous, 

 subtending an acute angle. Anterior end short, rounded, indis- 

 tinctly separated from the body by an obscure byssal sinus. Wing 

 large, triangular, joining the body of the valve near the posterior 

 extremity ; the limiting furrow is very distinct in young shells, and 

 obscure in older examples ; margin somewhat obliquely truncate ; 

 extremity not produced. 



Surface marked by fine radii, which are obscure or obsolete on 

 the anterior part of the shell ; also by concentric stria3, which are 

 somewhat acutely recurved on the angular portion of the body, and 

 in older shells are fasciculate, producing an undulated aspect. 



A small left valve has a length of 16 mm., height 11 mm., and 

 hinge-line 13 mm. Another specimen has a length of 30 mm., 

 height 18 mm., and hinge-line 25 mm. 



This species has some resemblance to P. alata in the left valve, but 

 its height is comparatively greater, the wing more truncate, and not 

 produced at the extremity. 



Formation and locality. In a shaly sandstone above a conglomerate 

 in the upper part of the Chemung group at Panama, Chautauqua 

 county, "N. Y. 



Ptychopteria elongata. 



Ptychopteria elongata, Hall. Pal. N. Y. , vol. v, pt. 1. Unpublished. 



Shell of medium size, sub-rhomboidal, elongate ; body sub-cylindrical, 

 oblique at an angle of 30° with the hinge-line; length more than 

 one-third greater than the height; anterior margin oblique, curving 

 into the basal margin, with a slight concavity for the byssal sinus; 

 posterior margin abruptly rounded. Left valve regularly convex 

 below, gibbous above. Right valve somewhat depressed-convex. 

 Hinge-line straight, about one-fifth less than the length of the 

 valve. Beaks near the anterior fourth of the hinge, directed for- 

 ward; left beak prominent, sub-acute, arching over the cardinal 

 line ; beak of right valve depressed, obtuse, scarcely rising above 

 the hinge-line. Umbo gibbous in the left valve ; the greatest con- 

 vexity of the valve is about one-third the length from the beak. 

 Umbonal angle acute. Anterior end short, produced and acute at 

 the extremity limited by a distinct oblique byssal depression. Wing 

 narrow-triangular, joining the body of the shell near the posterior 

 [Sen. Doc. No. 38.] 41 



