LAMELLIBRANCHIATA. 233 



straight and oblique below, recurving just beneath the hinge-line, and form- 

 ing a short mucronate extension. 



Test thin, marked by fine concentric striae which are irregularly crowded 

 into fascicles, producing an undulated aspect to the surface. The striae are 

 sub-lamellose on the anterior side, and sometimes fasciculate on the wing. 

 The fascicles on the body are often marked on their summits by sharp, 

 regular lamellae. The structure as seen in exfoliated specimens is radiated ; 

 but this does not appear to be an exterior character. 



The hinge is marked by a single narrow groove ; beyond this the charac- 

 ters of the interior are unknown. 



A large left valve has a length of 45 mm., height 25 mm., hinge-line 

 27 mm. A smaller specimen has a length of 28 mm., height 10 mm., 

 hinge-line 17 mm. A right valve has a length of 26 mm., height 15 mm., 

 and hinge-line 15 mm. 



Formation and localities. In the Chemung group at Philijjsburg, Alleghany 

 county, N. Y., and in Sullivan township, Tioga county, Pa. 



Leptodesma quadratum, n. sp. 



PLATE XCI, FIG. 15. 



Shell of medium size, rhomboidal or sub-quadrate ; body ovate, making an 

 angle of nearly 40° with the cardinal line ; length one-third greater than the 

 height ; ante-byssal margin nearly vertical in front, rounding below into the 

 shallow sinus, and extending nearly the whole height of the valve; ventral 

 margin nearly straight ; post-basal margin abruptly rounded and joining the 

 wing without limitation. 



Left valve convex, gibbous on the umbo, and obscurely angular along the 

 posterior slope. Right valve equally convex. 



Hinge-line straight, equal to three-fourths the length of the valve, and 

 equal to the height. 



Beaks situated at the anterior third of the hinge, prominent and arching 

 over the cardinal line. Umbonal region gibbous, subtending an acute angle. 

 30 



