132 PALEONTOLOGY OF NEW YORK. 



liiu's, which arc irregularly crowded into sub-imbricating Lamellae. On the 

 anterior end of the valve the radii arc cancellated by the concentric stria'. 



Interior characters unknown, except a narrow groove along the ligamental 

 area. 



A left valve lias a length of 41 mm., height 24 mm., hinge-line M2 mm. 

 A similar right valve lias a length of 40 mm., height 24 mm., hinge-line 

 •'! I nun. 



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. 

 ami thi' concentric lamella* 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. BIGS. 16, 23; kND PLATE LXXXV, FIGS. 14-K 



Pbycliapteria Sao. Hall. Pal. X. V.. vol. v. pt. 1. Plates and Explanations: PI. 23, figs. Hi, 33. 



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, genth curving into the 

 sinus; l>ase 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, 

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

 in the right, subtending an acute angle. 



