326 Thirty-fifth Eeport on the State Museum, 



acutely recurved at the basal angle, and the radii are finer and more 

 numerous. 



Formation and locality. In the- Upper Chemung group, the middle 

 beds of the series, as seen at Warren, Pa. 



Ptychopteria perlata. 



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 v, pt. 1, Unpublished. 



Shell large, rhomboidal ; body ovate, oblique ; length one-sixth 

 greater than the height ; anterior and basal margins broadly 

 curved; posterior end angular. Left valve very convex, gibbous 

 above. Eight valve somewhat gibbous above, having a con- 

 vexity of about one-half that of the left valve. Hinge-line 

 straight, nearly equal to the length of the shell. Beak at 

 about the anterior third of the cardinal line, prominent and 

 arching over the hinge in the left valve. Umbo gibbous in the left 

 valve and convex in the right, subtending an acute angle. Anterior 

 end large, obscurely defined by a shallow byssal depression, which 

 is obsolete in the right valve ; margin convex ; extremity acuminate. 

 "Wing broad-triangular, joining the body at the posterior end ; the 

 furrow and fold are stronger in the left valve ; margin slightly 

 convex below, inclining a little forward ; extremity produced, acute- 

 angular. 



Surface of the left valve marked by. fine, close, rounded radii, 

 which are often interrupted by the concentric lamellae. The radii 

 are finer upon the wing, obsolete upon the anterior end, and 

 scarcely present on the right valve. The concentric lines of growth 

 and lamellae give an irregularly undulating appearance to the 

 surface. 



A left valve has a length of 31 mm., height 26 mm., and hinge- 

 line 30 mm. A right valve measures 33 mm. in length, 27 mm. in 

 height, and hinge-line 32 mm. 



This species is a shorter, more erect form than P. lata, and the left 

 valve is more convex and broader below. 



Formation and locality. In the upper beds of the Chemung group 

 at Warren, Pa. 



