Lamellibranchiata. 325 



The specimen described has a length of 56 mm., height 30 mm., 

 and hinge-line about 48 mm. 



This species resembles P. lata, and also some forms of P. Sao, but 

 it is much more elongate, the valves more depressed, the wing nar- 

 rower and longer. 



Formation and locality. In the Upper Chemung group at Warren, 

 Pa. 



Ptychopteria lata. 



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 Unpublished. 



Shell large, rhomboidal ; body sub-elliptical, very oblique, making 

 an angle with the hinge of nearly 40° ; height equal to about two- 

 thirds of the length ; ante-byssal margin oblique, curving into 

 the shallow sinus, from whe*nce the basal margin is broadly rounded ; 

 posterior extremity recurved at a little less than a right angle. Left 

 valve more convex than the right and gibbous above, obscurely 

 angular along the post-cardinal slope. This difference in the valves 

 is also shown in the direction of the striae of growth. Hinge-line 

 straight, about three-fourths as long as the length of the shell. 

 Beaks near the anterior fourth of the hinge, prominent in the left 

 valve, acute, arching over the cardinal line ; in the right valve de- 

 pressed, rising only as high as the hinge. Umbonal region of the 

 left valve moderately gibbous, subtending an acute angle. Anterior 

 end short, small, limited by an oblique byssal depression which 

 extends along the base about one-third the length of the shell ; 

 extremity acute. Wing large, extending along the shell to near the 

 posterior extremity ; margin obliquely truncate, concave just below 

 the hinge-line, which is abruptly produced. 



Test thin, marked by fine, interrupted or undulated, radii, which 

 are more strongly marked on the body and wing, and obsolete on 

 the anterior of the valve ; the surface is also marked by fine con- 

 centric striae, which are crowded into fascicles at unequal distances, 

 undulating the surface ; they turn abruptly outward, just below 

 the hinge, and are lamellose on the anterior part of the valve. 



A specimen preserving both valves has -a length of 45 mm., 

 height 28 mm., hinge-line 35 mm. The right valve is somewhat 

 smaller. Another specimen has a length of 42 mm., height 28 

 mm., hinge-line 36 mm. 



This species resembles in form P. Sao, but is larger, the wing fur- 

 row more strongly defined on the left valve, the concentric lines more 



