328 Thirty-fifth Repobt on the State Museum. 



anterior to the middle of the hinge; very prominent in the left 

 valve and arching over the hinge. Umbonal region ample, very 

 gibbous, obscurely angular along the posterior slope, subtending an 

 acute angle. Anterior end somewhat large, gibbous, and separated 

 from the body of the shell by a nearly vertical sulcus ; extremity 

 acute and nasute. Wing of medium size, joining the body at the 

 posterior end ; the furrow and fold are very conspicuous, the fold 

 somewhat broader and nearly equal to the portion of the wing 

 above ; margin almost vertically truncate ; extremity apparently not 

 produced. 



Test thin, marked by fine radii and concentric striae. The con- 

 centric lines are crowded into fascicles, producing an undulated 

 appearance ; and are strongly lamellose on the anterior* end ; the 

 radiating striae are. distinct upon the body of the shell, obscure on 

 the wing, and obsolete on the anterior end. 



A specimen of the left valve has a length of 26 mm., height 15 

 mm., hinge-line 22 mm. Another has a length of 20 mm., height 

 13 mm., hinge-line 18 mm. 



This species is distinguished by the sub-central position of the beak 

 and the great prominence of the umbo, the marked byssal depression 

 and strong furrow and fold of the wing. 



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

 at Warren, Pa. 



PtYCHOPTEEIA LOB ATA. 



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 Unpublished. 



Shell small, sub-rhomboidal ; body broadly ovate, oblique at an angle 

 of about 55° with the cardinal line ; length somewhat greater than 

 the height; anterior margin sub -truncate;' base broadly rounded; 

 posterior margin rectangularly recurved into the wing. Left valve 

 very gibbous ; the point of greatest convexity is just below the umbo. 

 Right valve unknown. Hinge-line straight, length nearly equal to 

 the length of the valve. Beak situated anterior to the middle of 

 the hinge, prominent, inclined forward, and arching over the mar- 

 gin of the valve. Umbonal region ample, gibbous, subtending an 

 acute angle. Anterior end large, limited by a broad, undefined byssal 

 depression ; margin convex ; extremity apparently obtuse. Wing 

 small, joining the body at the posterior end, strongly limited by a 

 deep furrow and very marked alar plication ; margin nearly straight ; 

 extremity angular. 



