316 Thirty-fifth Report ox the State Museum. 



Ptychopteria Eucrate. 



Ptychopteria Eucrate, Hall. Pal. N. Y., vol. v, pt, 1. Plates and Explana- 

 tions ; PL 23, fig. 24. Jan., 1883. 



Shell above the medium size, rhomboidal ; body very narrow, elon- 

 gate-ovate, oblique at an angle of about 30° with the hinge-line ; 

 length nearly twice the height ; ante-byssal margin oblique, curving 

 into the sinus ; the ventral margin broadly curved ; posterior ex- 

 tremity abruptly or sub-angularly recurved. Left valve convex, 

 gibbous on the umbo and obtusely sub-angular along the post-cardi- 

 nal slope. Right valve less convex, somewhat smaller than the left. 

 Hinge-line straight ; length about two-thirds the length of the 

 valve. Beak near the anterior fourth of the length of the hinge, 

 directed forward ; beak of left valve prominent, acute^ incurved 

 over the hinge-line ; beak of right valve obtuse and not elevated. 

 Umbonal region of the left /valve gibbous and angular, of the right 

 valve convex, subtending a very acute angle. Anterior end small ; 

 margin rounded; extremity acute. Byssal depression oblique, 

 directed backward. Wing narrow-triangular, joining the body at 

 the posterior end; margin obliquely truncate, scarcely concave; ex- 

 tremity apparently not produced. 



Surface marked by fine radiating striae, which are more conspicu- 

 ous on the body of the valve and upon the wing, and are very ob- 

 scure on the anterior end: these are crossed by fine concentric striae 

 of growth which, at irregular intervals, are raised into fascicles, 

 usually more conspicuous on the umbo and post-cardinal slope. 

 On the right valve the radii are usually obsolete. Interior unknown, 

 except the usual linear groove along the hinge-line. . 



A large left valve has a length of 40 mm., height 20 mm., and 

 hinge-line 28 mm. Another is 38 mm. in length, height 20 mm., 

 and hinge-line 27 mm. 



In this species the body is more oblique than in P. Salamanca; the 

 angularity of the left valve is in a direct line down the middle of the 

 body; the shell is comparatively narrower; the extremity of the wing 

 is not produced ; the byssal depression is more oblique. The want of 

 striae on the anterior end and their obsolescence on the right valve are 

 marked characters. It is proportionally much longer than P Sao, 

 with which it is associated. 



Formation and locality. In the lower part of a conglomerate of the 

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



