L36 I'AL.EONTOLOGY OF NEW YORK. 



Ptvi IHOPTERU FALCATA, 11. sp. 



PL Ml. I WW . FIGS. 6, 7. 



Shell of medium size, rhomboidal, sub-falcate •; body narrow-ovate, oblique at 

 an angle of about 45° with the cardinal line; length one-fourth greater than 

 the height; anterior and basal margins broadly rounded ; posterior margin 

 abruptly recurved. 



Left valve convex, gibbous above, arcuate. Right valve convex on the 

 umbo, depressed-convex In-low. 



Hinge-line straight, greater than the height of the shell. 



Beak situated at about the anterior fourth of the cardinal line, prominent 

 and arching over the hinge in the left valve: not elevated in the right valve. 

 Umbo abruptly gibbous in the left valve, subtending an acute angle. 



Anterior end limited by a shallow, nearly vertical byssal depression, which 

 inclines backward in some specimens; margin convex: extremity acuminate. 

 Wing triangular, joining the body near the posterior end : margin straight, 

 slightly concave just below the hinge-line; extremity acute. 



Surface of both valves marked by line radiating striae, which are less 

 conspicuous on the wing and obsolete on the anterior end : and with 

 concentric stria>, which are sometimes strongly lamellose, producing undu- 

 lations of the surface. 



Interior unknown. 



A left valve has a length of li* mm., height 'I'l nun., hinge line 24 mm. 

 \ similar right valve has a length of 28 mm., height 15 mm., and hinge-line 

 -\ mm. 



The shell of this .species is comparatively longer, the body more oblique, and 

 the wing smaller than in P. Thetis. 



Formation and locality. In a sandstone and conglomerate of the Upper 

 Chemung group at Alleghany Springs, Warren county, Pa. 



