LAMELLIBRANCHIATA. 131 



A large specimen of the left valve has a length of 31 mm., height 19 mm., 

 and hinge-line 25 mm. A smaller one has a length of 24 mm., heiglit 13 

 mm., hinge-line 20 mm. 



This species has a narrower anterior end than the preceding, a larger and 

 more extended wing, finer and undulating stria?. 



Formation and localities. In sandstones of the Upper Chemung group, Smeth- 

 port, McKean county, and at Warren, Warren county, Pa., associated with 

 Spirifera Verneuili. 



Ptychopteria Salamanca. 



PI.-VTE XXUI, FIGS. 17-in. 



Ptychopteria Salamanca, Hall. Pal. N. Y.. vol. v, pi. 1. Plat.'s :niil Kx]>laiiati<)iis : PI. 2S, tiss. 17-'2(). 



.Ian.. ISSH. 



Shell large, rhomboidal ; body sub-ovate, oblique at an angle of about 40*^ with 

 the hinge-line; length more than once and a half the lieiglit; unte-byssal 

 margin oblique, rounded below, slightly concave at the sinus, and sloping to 

 the broadly rounded base, thence abruptly recurved and continued almost 

 vertically into the margin of the wing. 



Valves convex, gibbous above the middle. Tlie left valve is more gibbous 

 than the right and somewhat larger. 



Hinge-line straight, length more than three-fourths the length of the shell. 



Beaks in front of tlie anterior third of the cardinal line, directed forward; 

 the left beak prominent, acute, arching over the hinge ; the right beak obtuse, 

 depressed, scarcely rising above the hinge-line. Umbonal region gibbous, 

 subtending an acute angle. The umbo and beak of the left valve are much 

 more prominent, and the post-cardinal slope is obtusely sub-angular. 



Anterior end comparatively small, triangular, produced into an acute 

 extension. Wing large, triangular ; margin gently concave ; extremity pro- 

 duced, acute. The fold and sulcus are more conspicuous in the right valve. 



(Surface marked by fine, i-egular, uiuhdating and interrupted radii, which 

 are more conspicuous on the wing: crossing these are regular concentric 



