LA MEL LIB RANCHI A TA. 
143 
Interior unknown. 
A left valve has a length of 26 mm., height 18 mm., and hinge-line 20 mm. 
Another specimen has a length of 24 mm., height 14 mm., and hinge-line 
17 mm. 
Compared with P. Eucrate and P. elongata, the body of the shell is compara¬ 
tively shorter, the furrow of the wing more distinct, the byssal depression 
shallower and more nearly vertical. It has the obscure angularity of P. Eucrate , 
with coarser and more nearly vertical radii. 
Formation and locality. In the lower beds of the Upper Division of the 
Chemung group, as seen at Warren, Pa. 
Ptychopteria Beecheri, n. sp. 
PLATE LXXXV, FIGS. 21, 22, 
Shell large, rhomboidal; body cylindrical, very arcuate in the left valve, oblique 
at an angle of about 40° with the hinge; length nearly twice the height; 
ante-byssal margin slightly oblique, curving 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 eleva¬ 
tion is continued into the angular, posterior extremity. Right valve much 
less convex. 
Hinge-line straight, length more than two-thirds the length of the shell. 
Beaks at about the anterior 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; 
