318 Thirty-fifth Report on the State Museum. 



gibbous above, arcuate. Right valve convex on the umbo, depressed- 

 convex below. 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 shal- 

 low, nearly vertical byssal depression, which inclines backward in 

 some specimens; margin convex ; extremity acuminate. Wing tri- 

 angular, joining the body near the posterior end ; margin straight 

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



Surface of both valves marked by fine radiating striae, which are 

 less conspicuous on the wing and obsolete on the anterior end; and 

 with concentric striae, which are sometimes strongly lamellose, pro- 

 ducing undulations of the surface. 



A left valve has a length of 28 mm., height 22 mm., hinge-line 

 24 mm. A similar right valve has a length of 28 mm., height 15 

 mm., and hinge-line 24 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. 



Ptychopteria Spio. 

 Ptychopteria Spio, Hall. Pal. N. T., vol. v, pt. 1. Unpublished. 



Shell of medium size, rhomboidal ; body elongate-ovate, oblique to 

 the hinge-line at an angle of about 30°; length considerably more 

 than one-third greater than the height ; ante-byssal margin curving 

 into a shallow sinus ; ventral margin broadly rounded ; posterior 

 margin abruptly recurved. Left valve regularly and moderately 

 convex, gibbous on the umbo. Right valve unknown. Hinge-line 

 straight, greater than the height of the valve. Beak in front of the 

 anterior third, moderately prominent and scarcely rising above the 

 hinge. Anterior extremity narrowly acute. Wing very obliquely 

 truncate, joining the body near the posterior extremity ; the furrow 

 and fold separating it from the body are only moderately developed ; 

 extremity not produced. 



Test thin, marked by fine undulating radii which are obsolete on 

 the anterior side; also by concentric striae which are prominent and 

 lamellose on the anterior. 



The specimen described has a length of 35 mm., height 20 mm., 

 hinge-line 24 mm. 



