Lamellibranchiata. 317 



Ptychopteria Thetis. 



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 Unpublished. 

 Shell of medium size, rhomboidal ; body narrow-ovate, oblique at 

 an angle of about 50 ° to the hinge-line; length one-third greater 

 than the height; ante-byssal margin obliquely sub-truncate, and 

 curving into a very shallow sinus, from which it makes a broad 

 curve to the sub-angular posterior margin, whence it is abruptly re- 

 curved. Left valve convex below, gibbous above, very obtusely sub- 

 angular along the middle. Hinge-line straight, length a little 

 greater than the height of the valve. . Beak at about the anterior 

 third of the hinge, prominent, arching over the cardinal line 

 and directed forward. Umbonal region gibbous, subtending an acute 

 angle. Anterior end short, limited by a nearly vertical byssal de- 

 pression ; extremity acute. Wing of medium size, triangular, join- 

 ing the body of the shell nearly at the posterior extremity ; margin 

 truncate, oblique ; extremity apparently not extended. 



Surface marked by concentric strise, which are irregularly fascicu- 

 late, and have an angular recurvation along the middle of the body. 

 The body and wing are marked by comparatively fine radii, which 

 are sometimes distinctly cancellated by the concentric lines, and are 

 obscure or obsolete on the anterior portion of the shell, which is 

 marked only by the lamellose concentric strise. 



A specimen of the left valve of the usual size has a length of 32 

 mm., height 21 mm., and hinge-line 22 mm. 



This species resembles P. Sao> but the angle of the body to the 

 hinge-line is greater, and from the umbo to the posterior end, the 

 valve is sub-angular ; the hinge is longer in proportion, and the fold 

 of the wing less conspicuous. A right valve in the same association 

 has about the same proportions, but the body is narrower and the 

 radii coarser, making the identity doubtful. 



Formation and locality. In a conglomerate of the Chemung group 

 at Panama, Chautauqua county, N. Y. 



Ptychopteria falcata. 

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 Unpublished. 



Shell of medium size, rhomboidal, sub-falcate ; body narrow-ovate, 

 oblique at an angle of about 45° with the cardinal line ; length one- 

 fourth greater than the height; anterior and basal margins broadly 

 rounded ; posterior margin abruptly recurved. Left valve convex, 



