22 1 PALEONTOLOGY OF NEW YORK. 



wing nl the right valve is scarcely defined from the body and less deeply 

 concave mi the margin. 



Tesl marked by line, irregular, concentric striae which are strongly fascicu- 

 late and lamellose mi the ventral side and anterior end, and are shown on 

 the wing as sharp, elevated, regular striae. Some specimens preserve fine 

 regular stria' on the body of the valve, but they are usually somewhat irreg- 

 ular and lamellose. In an exfoliated and macerated condition they show- 

 line radiating lines which probably belong to the intimate structure of the 

 shell. 



Ligamental area (as preserved in an imperfect right- valve) 1.5 mm. wide. 

 and marked by very fine, minutely undulating longitudinal strice. The 

 hinge also shows an obscure oblique lateral tooth. 



A left valve, of medium size, has a greatest length of 56 mm., height 

 31 mm., and hinge-line about 51 mm. 



This species somewhat resembles L. aviforme, but is distinguished by its more 

 oblique form, and wider anterior end. Specimens are quite abundant in an 

 argillo-calcareous sandstone of the Upper Chemung group, associated with 

 Spirifera Verneuili. 



Formation and locality. In the Upper Chemung group, Auburn township, 

 Susquehanna county, Pa. 



Leptodesma aviforme, n. sp. 



PLATE XCI, PIG. S 



Shell large, sub-rhomboidal ; body elongate-ovate, broad behind, and rapidly 

 narrowing toward the beak, oblique at an angle of a little less than 45° to ' 

 the hinge-line; height more than one-half the length ; ante-by ssal margin 

 ver\ oblique, continued in a straight line into the shallow sinus, thence 

 curving to the posterior extremity, which is broadly rounded. 



Left valve gently convex below, very convex above, and moderately 

 gibbous on the umbo. Right valve unknown. 



