2:^2 PALEONTOLOGY OF NEW YORK. 



extremity, undefined: niar<:in straight or slightly concave, very oblique; 

 extremity slightly produced, angular. 



Test marked by regular, even, concentric striie, 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 \:\\\(' has a greatest length of 45 mm., height 20 mm., 

 ami hinge-line 26 iniii. A similar I'ight valve has a length of 37 mm., 

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



This species is very similar to L. Lichas, but dill'ers in the greater convexity 

 of body, undefined Ijyssal depression, and narrower wing. 



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

 field, Tioga comity, Pa. 



Leptodesma Lichas. 



PLATli XXI, FIGS. 35-39; AXn PLATE XCI, FItiS I'J, 20. 



LrptodesiiM Lichas, Hai.l. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 21, fig-.*. 3.5-39. 



.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 abruptl}- recurved. 



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

 making the body somewhat cylindrical. Right \al\c probably a little 

 shorter tlian 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 bys.sal sinus 

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

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



