Lamellibranchiata. 315 



Pttchopteria Sao. 



PtycTwpteria Sao, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 

 PI. 23, figs. 16, 23. Jan., 1883. 



Shell of medium size, rhomboidal ; body narrow, elongate-ovate, 

 oblique at an angle of about 45° with the hinge-line; length one- 

 third greater than the height ; ante-byssal margin obliquely sub-trun- 

 cate, gently curving into the sinus ; base broadly rounded ; posterior 

 margin extended, abruptly recurved to the wing. Valves convex ; 

 left valve larger and more convex than the right. Hinge-line 

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

 anterior third of the cardinal line, obtuse, prominent in the left 

 valve and arching over the hinge; less prominent in the right valve, 

 and rising just above the hinge. Umbo gibbous in the left valve, 

 less gibbous in the right, subtending an acute angle. Anterior end 

 short, small, acute, limited by a shallow byssal depression. Wing 

 large, triangular, extending to near the posterior extremity of the 

 body ; margin slightly oblique, scarcely concave ; extremity appar- 

 ently not produced. 



Surface marked by fine radii which are more or less interrupted 

 by the varices of the concentric striae, and are frequently undulating. 

 The concentric striae are often fasciculate, and in their usual con- 

 dition give an undulated appearance to the cast, but where well-pre- 

 served they are angular and somewhat lamellose. Interior unknown, 

 except a narrow groove along the ligamental area of the hinge. 



A left valve of usual dimensions has a length of 30 mm., height 

 19 mm., hinge-line 23 mm. A similar right valve has a length of 

 31 mm., height 18 mm., and hinge-line 22 mm. 



This species differs from P. Proto in its less angular posterior slope 

 and posterior end ; the margin of the wing is less oblique to the hinge- 

 line and the extremity is not produced. The specimens occurring in a 

 conglomerate or coarse sandstone have been subjected to different de- 

 grees of pressure and maceration, and consequently present a great 

 variety of aspect in the surface ornamentation. The furrow and fold 

 limiting the wing are always less conspicuous in the left valve ; and in 

 some cases when the shell has suffered pressure these characters are 

 very obscure. 



Formation and locality. Abundant in a conglomerate of the Che- 

 mung group at Panama, Chautauqua county, N. Y. 



