L92 PALEONTOLOGY OF NEW YORK. 



Leptodesma BlLLINGSI, 11. sp. 



PL VTE I.WXIV FIGS 12-15 



Shell large, sub-rhomboidal ; body elongate-ovate, or sub-cylindrical, very 

 oblique; length about twice the height; ante-byssal margin rounded, 

 becoming straight or slightly concave at the sinus; ventral margin broadly 

 curving; posterior margin extended, acutely recurved at the post-basal 

 extremity. 



Left valve gibbous on the umbo, convex below. Right valve unknown. 



Hinge-line straight : length, including the spine, nearly equal to the length 

 of the valve. 



Beak sub-anterior, obtuse, slightly oblique, prominent. Umbonal region 

 somewhat gibbous, subtending an acute angle. 



Anterior end limited by a shallow byssal depression, acute and nasute at 

 the extremity. Wing long, narrow-triangular, extending below the middle 

 of the valve; margin sinuate: extremity produced into a spine. 



Test thin, marked by tine concentric stri;e which are abruptly arched on 

 the posterior slope, regular upon the wing, and crowded into tine irregular 

 fascicles which are conspicuous on the anterior. Some specimens show sharp, 

 elevated, laiuellose stri;e at equal distances on the surface of the body. The 

 surface characters vary with the different states of exfoliation and maceration. 



The largest specimen observed has a greatest length, from beak to base, 

 of 57 mm., height 36 mm., binge-line, to base of spine, 35 mm. A small 

 example of the left valve has a length of 34 mm., height IS mm., and hinge- 

 Line about 35 mm. 



This species is more elongate and oblique than L. longispinum, the wing extends 

 farther down along the posterior slope, and the anterior extremity is fuller and 

 more acute. 



Formation and localities. In the Chemung group at Panama, N. V. Other 

 specimens from near Elmira, N. V., and Mansfield. Tioga county. Pa., are 

 referred to this species with reservation. 



