230 PALAEONTOLOGY OF NEW YORK. 



Hinge-line straijrht ; length about equal to the height of the shell. 



Beaks prominent, directed forward and arching over the hinge, situated 

 at about the anterior third of the cardinal line. Uinl)onal region angularly 

 gibbous al)Ove, alnniptly defined along the posterior slojje. Umbonal angle 

 acute. 



Anterior end short, angular. Wing an uudeliued, narrow-triangular 

 expansion; margin straight or a little concave; extremity very slightly 

 produced. 



Test moderately thick, marked l)y fine concentric striae, which are 

 fasciculate on the body of the shell, and crowded ami laraellose over the 

 anterior side. 



Interior unknown. 



A large left valve has a length of 00 mm., height 36 mm., and hinge- 

 line 37 nun. 



This species is distinguished from L. Lichas by its larger and stronger form, 

 being very gibbous along the entire length of the shell, and with a propor- 

 tionally smaller wing. 



Formation and locality. In the upper part of the Chemung group at Mansfield, 

 Tioga county. Pa. 



Leptodesma Phaon, n. sp. 



PLATE XCI, FIG. 18. 



Shell above the medium size, narrow-rhomboidal ; body sub-cylindrical, very 

 oblique, making an angle with the hinge-line of less than 30° ; length nearly 

 twice the height ; ante-byssal margin short, oblique, curving into the broad, 

 almost straight ventral side ; posterior margin abruptly but regularly rounded 

 and continuous with the base of the wing. 



Left valve very convex, gibbous above. Right valve unknown. 



Hinge-line straight, somewhat longer than the height of the .shell. 



Beak at the anterior third of the hinge-line, obtu.^e, moderately elevated, 

 directed forward. Umbonal region gibbous, not well-defined anteriorly, 

 abruptly depressed on the posterior. Umbonal angle acute. 



