132 
PALEONTOLOGY OF NEW YORK. 
lines, which are irregularly crowded into sub-imbricating lamellae. On the 
anterior end of the valve the radii are cancellated by the concentric striae. 
Interior characters unknown, except a narrow groove along the ligamental 
area. 
A left valve has a length of 41 mm., height 24 mm., hinge-line 32 mm. 
A similar right valve has a length of 40 mm., height 24 mm., hinge-line 
34 mm. 
This species varies in different conditions of preservation. The obtusely 
sub-angular appearance of the posterior slope is often exaggerated by lateral 
pressure or subdued by vertical pressure. The same is true of the folds of the 
wing. In some conditions the radiations are continuous, in others they are 
cancellated. In weathered specimens the striae are sometimes nearly obsolete, 
and the concentric lamellae more prominent. 
Formation and locality. In the Chemung group, in a sandstone above the 
conglomerate at Rock City, near Salamanca, N. Y. 
Ptychopteria Sao. 
PLATE XXIII, FIGS. 16, 23 ; AND PLATE LXXXV, FIGS. U-18. 
Ptychopteria /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-truncate, 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 h-inge. Umbo gibbous in the left valve, less gibbous 
in the right, subtending an acute angle. 
