Lamellibranchiata. 323 



A left valve has a length of 26 mm., height lb mm., and hinge- 

 line 20 mm. Another specimen has a length of #4 mm., height 14 

 mm., and hinge-line 17 mm. 



Compared with P. Eucrate and P. elongata, the body of the shell 

 is comparatively shorter, the furrow of the wing more distinct, the 

 byssal depression shallower and more nearly vertical. It has the 

 obscure angularity of i 5 , Eucrate, with coarser and more nearly verti- 

 cal radii. 



Formation and locality. In the lower beds of the Upper Division 

 of the Chemung group, as seen at Warren, Pa . 



Ptychopteria Beecheri. 



Ptychopteria Beecheri, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell large, rhomboidal; body cylindrical, very arcuate in the left 

 valve, oblique at an angle of about 40 9 with the hinge ; length 

 nearly twice the height ; ante-byssal margin slightly oblique, curv- 

 ing gently into the sinus; ventral margin broadly rounded, 

 straighter toward the posterior end which is acutely recurved. Left 

 valve ventricose, the greatest convexity a little above the middle ; 

 abruptly rounded or sub-angular along the post-cardinal slope, and 

 the elevation is continued into the angular posterior extremity. 

 Eight valve much less convex. Hinge-line straight, length more 

 than two-thirds the length of the shell. Beaks at about the ante- 

 rior third of the hinge-line, very prominent and incurved over the 

 hinge in the left valve. In the right valve the beak is subdued, 

 not rising above the hinge-line. Umbo very gibbous in the left 

 valve, subtending an acute angle. Anterior end moderately large, 

 limited by a very broad, shallow, slightly oblique byssal depression ; 

 extremity flattened, acute, defined by an oblique constriction in the 

 margin just below the cardinal line. Wing of medium size, joining 

 the body of the shell a little above the posterior extremity ; the 

 limiting furrow distinct; the fold gentle and the upper part of the 

 wing flat or slightly convex; margin obliquely truncate, concave 

 just below the hinge; extremity slightly produced. 



Surface marked by fine uniform radii on the body and wing, which 

 are nearly obsolete on the anterior end, and sometimes interrupted 

 on the body by varices of growth ; also by fine concentric striae 

 which are acutely recurved over the angular posterior slope of the 

 body, and crowded into fascicles at irregular intervals, producing an 

 undulated appearance. 



A large left valve has a length of 52 mm., height 28 mm., and 



