Lamellibranchiata. 327 



Ptychopteria Thalia. 

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 Unpublished. 



Shell of medium size, rhomboidal ; body ovate, oblique at an angle 

 of about 45° with the cardinal line ; length one-third greater than 

 the height; anterior and basal margins broadly rounded, slightly 

 impressed by the byssal sinus ; post-basal extremity sub-angular, 

 abruptly recurved. Left valve extremely gibbous, the point of 

 greatest convexity is one-third of the height from the beak. Eight 

 valve convex on the umbo, depressed-convex below. Hinge-line 

 straight, a little less than the greatest length of the shell. Beak 

 situated at the anterior third of the hinge, prominent in the left 

 valve, curving over the hinge-line. Umbonal region abruptly 

 gibbous in the left valve, distinctly limited, subtending an acute 

 aiigle. Anterior end comparatively large, limited by a nearly verti- 

 cal shallow byssal depression ; margin convex ; extremity acute. 

 Wing small, distinctly limited by the post-umbonal furrow \ margin 

 obliquely truncate, a little convex, extremity obtuse-angled. 



Surface of both valves marked by fine radii originating on the 

 umbo, becoming stronger on the lower part of the body, less con- 

 spicuous on the wing, and obsolete on the anterior end ; and by 

 lines of growth which are lamellose on the anterior side and pro- 

 duce unequal concentric ridges on the body of the valve. 



A left valve has a length of 25 mm., height 17 mm.; and hinge- 

 line 23 mm. A small right valve has a length of 20 mm., height 

 13 mm., and hinge-line 16 mm. 



This species resembles P. ])erlata but the wing is comparatively 

 smaller, extremity not acuminate, and the umbo more gibbous. 



Formation and locality. In a sandstone of the Upper Chemung 

 group at Alleghany Springs, Warren county, Pa. 



Ptychopteria gibbosa. 



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 Unpublished. 



Shell small, rhomboidal ; body broad-ovate, oblique at an angle of 

 about 45° with the hinge-line; length not more than one-seventh 

 greater than the height; ante-byssal margin oblique, curving into 

 the marked sinus, thence along the base to the nearly rectangularly 

 recurved posterior extremity. Left valve very convex, gibbous on 

 the umbo and in the middle. Eight valve unknown. Hinge-line 

 straight, nearly as long as the length of the shell. Beaks a little 



