224 PALEONTOLOGY OF NEW YORK. 



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

 concave on the margin. 



Test marked by fine, irregular, concentric striae which are strongly fixscicu- 

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

 tlie wing as sharp, elevated, regular striae. Some specimens preserve line 

 resrular striae on the lioilv of tlie valve, ])ut tlic\' are usuallv somcwliiit irreg- 

 ular and lamellose. In ;iii cxIoIiattMl and macerated conditi(Jii tlicy sliow 

 fine radiating lines which probably l)elong to the intimate structure of the 

 sliell. 



Ligamental area (as preserved in an imperfect right valve) l.o mm. wide, 

 and marked by very fine, minutely undulating longitudinal striie. 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. avifnrme, but is distinguislied by its more 

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

 argillo-calcareous sandstone of the Upper Chemung group, associateij witli 

 Spirifera Verneuili. 



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

 Susquehanna county. Pa. 



Leptodesma avifokme, n. sp. 



I'l.ATI-; XCI, KIG. 8. 



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

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

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

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

 curving to the posterior extremity, which is broadly rounded. 



Left valve gentlj- convex below, very convex above, and moderately 

 gibbous on the umbo. Right valve unknown. 



