232 PALAEONTOLOGY OF NEW YORK. 



extremity, undefined; margin straight or slightly concave, very oblique; 

 extremity slightly produced, angular. 



Tesl marked by regular, even, concentric striae, which, when well preserved, 



are elevated and lamellose. and are crowded and fasciculate on the anterior 

 end of the shell. 



Interior unknown. 



A nearly entire left valve has a greatest length of 45 mm., height 20 mm., 

 and hinge-line 26 mm. A similar right valve has a length of 37 mm., 

 height 18 mm., and hinge-line 22 mm. 



This species is very similar to L. Lichas, but differs in the greater convexity 

 of body, undefined byssal depression, and narrower wing. 



Formation and locality. In the middle portion of the Chemung group at Mans- 

 field, Tioga county. Pa. 



Leptodesma Lichas. 



PLATE XXI, PIGS. 35-39; axd PLATE XCI, FIGS. in, iO. 



Lcpt orlrx ma Lichas, Hall. Pal. N. V.. vol. v, pt. 1. Plates and Explanations: PL 21, figs. ;io-39. 



Jan., 1883. 



Shell of medium size, sub-rhomboidal ; body sub-cylindrical, very oblique ; 

 length nearly twice the height ; ante-byssal margin subtruncate, extending 

 below to the broad, gently curving base ; posterior margin abruptly recurved. 



Valves very convex above, gently convex toward the posterior extremity, 

 making the body somewhat cylindrical. Right valve probably a little 

 shorter than the left, as inferred from the separated valves. 



Hinge-line straight, less than the length of the shell. 



Beaks acute, situated at the anterior third of the hinge-line, directed for- 

 ward, prominent, arching over the cardinal line. Umbo abruptly gibbous, 

 subtending an acute angle. 



Anterior end large, acute and nasute at the extremity. The byssal sinus 

 makes only a slight depression in the margin. Wing narrow-triangular, join- 

 ing the body near the posterior extremity, not distinctly limited ; margin 



