LAMELLIBRANCHIATA. 143 



Interior unknown. 



A left valve has a length of 26 mm., height 18 mm., and hinge-lirie 20 mm. 

 Another sioecimen has a length of 24 mm., height 14 mm., ami liinge-line 

 17 mm. 



Compared with P. Eucrate and P. elongata, the l)ody 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. 



Ptychopterta Beeoheri, 11. sp. 



I'LATE LXXXV, KIGS. '21, 22. 



Shell large, rhomljoidal ; body cylindrical, very arcuate in the left valve, oblique 

 at an angle of aliout 40° with the hinge ; length nearly twice the height ; 

 ante-byssal margin slightly oblique, curving gently into the sinus ; ventral 

 margin broadh' roundeil, straighter toward the posterior end, which is 

 acutely recurved. 



Left valve ventricose, the greatest convexity a little iibove 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, lengtli moi-i' than two-thirds the length of the shell. 



Beaks at about the anterior third of the hinge-line, very prominent and 

 incurved over the liinge in the left valve. In the right valve the beak is 

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

 vfilve, sul)tending an acute angle. 



Anterior end moderately large, limited by a very Ijroad, shallow, slightly 

 oblique byssal depression ; extremity flattened, acute, defined by an o1)li([ue 

 constriction in the margin just l)olow the cardinal line. Wing of medium 

 size, joining the hody of tlu' shell a little above the posterior extremity ; 



