I is PALAEONTOLOGY OF NEW YORK. 



Ptychopteri a Thalia, n. sp. 



PI \TE I. XXXV. FIGS 33, 38. 



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. Right 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 angle. 



Anterior end comparatively large, limited by a nearly vertical 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 tine radii originating on the umbo, 

 becoming stronger on the lower part of the body, less conspicuous on the 

 wing, and obsolete on the anterior end; and by lines of growth which 

 are lamellose on the anterior side and produce unequal concentric ridges on 

 the body of the valve. 



Interior unknown. 



A left valve has a length of 2o 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 lii nun. 



This species reseml lies P. perlata but the wing is comparatively smaller, 

 extremit) not acuminate, and the umbo more gibbous. 



Formation and locality. In a sandstone of the Upper Chemung group at Alle- 

 ghany Springs, Warren county, Pa. 



