LAMELLIBRANCHIATA. 77 



striae. The radii upon the right valve are also stronger ami distinctly bifurcat- 

 ing; the concentric striae are stronger, closer, and crenulate the rays more dis- 

 tinctly. P. suborbicularis is less oblique and the form more rounded, while 

 the rays are more numerous and closely arranged. 



Formation and localities. In the Chemung group at Belmont, Alleghany county, 

 N. Y., and Mansfield, Tioga county, Pennsylvania. 



Pterinopecten erectus. 



PLATE I.XXXII, FIGS [5-17. 



Shell above medium size, transversely semi-elliptical, scarcely oblique; length 

 and height nearly equal ; pallia! margins regularly rounded, slightly ex- 

 tended on the posterior side. 



Valves moderately convex. Right valve apparently more convex than the 

 left ; the differences between them have not been satisfactorily determined. 



Hinge-line straight, length a little greater than the length of the shell. 



Beak small, acute, directed forward, little elevated above the hinge- 

 margin ; anterior to the middle of the shell. Umbonal region not defined. 



Ears triangular, unequal. Posterior ear large, undefined; margin slightly 

 concave ; extremity acute. Anterior ear much smaller, not defined ; margin 

 concave ; extremity acute. Byssal sinus shallow. 



Surface marked by numerous rounded unequal radii ; the larger ones 

 alternating with one, two or three smaller ones; crossed by fine, sharp, 

 regular, concentric striae of growth. On the ears the striae are fine and 

 uniform. 



Interior not known. Ligamental area narrow. 



The largest specimen has a length of 32 mm., height 30 mm., hinge-line 

 about 34 mm. 



There is some resemblance between this species and P. dispandus, but it 

 differs in its comparatively smaller size, longer binge-line, and somewhat less 

 oblique form.. 



Formation and locality. In the shales of the Chemung group at a point 600 

 feet above the base of the formation, at Ithaca, N. Y. 



