202 PALEONTOLOGY OF NEW YORK. 



Anterior end small, terminating in a narrow uasute extension. Wing 

 comparatively large, triangular, joining the body near the posterior end; 

 surface concave: margin gently sinuate; extremity produced into a short 

 spiniform extension. 



Tesl thin, marked by fine concentric striae, which are somewhat regularly 

 fasciculate on the hodv of the shell, crowded and sub-lamellose on the 

 anterior, and regular over the wing, curving backward just below the hinge, 

 indicating a spiniform extension of the wing. On the body of the right 

 valve the striae form sharp, lamellose elevations at somewhat equal intervals. 



The hinge is marked by one or two narrow, longitudinal grooves. 



A left \ al\ e has a length of 34 mm., height 24 mm., and hinge-line 22 mm. 

 A similar specimen, somewhat vertically compressed, has a length of 36 mm., 

 height 24 mm., hinge-line 25 mm. 



This species resembles L. umbonatum var. depressum, but differs in its smaller 

 and narrower anterior end and more cylindrical body, and wing without a broad 

 sinus in the margin. 



Formation and localities. In the upper beds of the Chemung group, Steuben 

 county, N. Y., and Tioga and Bradford counties, Pa. 



Leptodesma Creon, n. sp. 



PLATE \i . FIGS U-13. 



Shell below medium size, sub-rhomboidal ; body ovate, oblique to the hinge- 

 line at an angle of about 55°; length nearly one-third greater than the 

 height; ante-byssal margin slightly oblique or rounded, distinctly sinuate 

 below, then gently curving to the broad l>ase: posterior margin broad, join- 

 ing the wing without interruption. 



Left valve convex below, gibbous above. Right valve less gibbous than 

 tin- left and more expanded. 



Hinge-line straight : length a little greater than the height of the shell. 



Beaks at about the anterior third of the hinge-line, sub-acute, directed 

 forward, prominent, arching over the hinge. Umbonal region very gibbous. 



