LAMELLIBRANCHIATA. 135 



Ptychopteria Thetis, n. sp. 



PLATE LXXXV, FIGS. 1-4. 



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 abrujjtly recurved. 



Left valve convex below, gil)bous 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 li]ie and directed forward. Umbonal region gibbous, subtending an 

 acute angle. 



Anterior end short, limited by a nearly vertical byssal depression ; 

 extremity acute. Wing of medium size, triangular, joining the l)ody of the 

 shell nearly at the posterior extremity ; margin truncate, o1)lique ; extremity 

 ap^iarently not extended. 



Surface marked by concentric stria), which are irregularly fasciculate, 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 oni}' by the lamellose concentric stria,'. 



Interior unknown. 



A specimen of the left valve of the usual size has a length of o2 mm., 

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



This species resembles P. Sao, Inxt the angle of the body to the hinge-line is 

 greater, and from the uml)o to the posterior end, the valve is sul)-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. 



