Lamellibranchiata. 3?3 



greatly extended, acute, rostrate, limited by a shallow byssal de- 

 pression. Wing narrow- triangular, joining the body at the posterior 

 extremity; defined by the recurving of the striae; margin abruptly 

 and symmetrically concave, extremity produced beyond the posterior 

 limit of the shell. 



Test thin, marked by fine, sharp, closely arranged, concentric striae, 

 sometimes fasciculate upon the body and anterior side, and very 

 distinct upon the wing. Partial casts of the interior of the left 

 valve give indications of fine radii which appear to belong to the 

 intimate structure of the shell. 



A left valve has a length of 32 mm., height 16 mm., and hinge- 

 line 37 mm. A right valve has a length of 34 mm., height 19 mm., 

 and hinge-line 40 mm. 



This species is distinguished by the great length of the hinge-line, 

 its narrow outline, and anterior extension. It approaches Pterostites 

 in character, but diifers in having a distinct wing, more depressed 

 form, and a recurvation of the striae along the post-umbonal slope. 



Formation and localities. In the Chemung group, Philipsburg, 

 Alleghany county, N. Y., and Kelly's creek, Tioga county, Pa. 



Leptodesma Hector. 



Lepiodesma Hector, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell of medium size, sub-rhomboidal ; body narrowly elongate- 

 ovate, oblique at an angle of about 30° with the cardinal line ; 

 length a little more than once and a half the height ; ante-byssal 

 margin oblique, curving into a broad, shallow sinus; ventral mar- 

 gin broadly curved ; posterior margin abruptly recurved. Left 

 valve moderately convex below, narrowly gibbous above. Eight 

 valve depressed below, gently convex above. Hinge-line straight ; 

 length equal to about four-fifths of the length of the valve. Beak 

 sub-anterior, directed forward, acute, slightly elevated above the 

 hinge. Umbonal angle very acute. Anterior end narrow, small, 

 produced into an acute extension, defined by a shallow byssal de- 

 pression. Wing large, joining the body at three-fourths or more of 

 its length from the beak ; margin very oblique below, deeply sinuate 

 just beneath the cardinal line, turning abruptly backward and 

 terminating in a spiniform extension. The wing of the right valve 

 is scarcely defined, and the shell has a somewhat broadly spatulate 

 aspect. 



Test thin, marked by fine, close, concentric striae, and, as usually 

 seen, the surface shows nearly equally distant lamellose striae on the 



