Lamellibranchiata. 385 



basal extremity, limited by the abrupt forward bending of the striae ; 

 margin deeply concave ; extremity produced and acute. The 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 

 fasciculate and lamellose on the ventral side and anterior end, and 

 are shown on the wing as sharp, elevated, regular striae. Some 

 specimens preserve fine regular striae on the body of the valve, but 

 they are usually somewhat irregular and lamellose. In an exfoliated 

 and macerated condition they show fine 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 striae. 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. avifor?ne, 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 ayiforme. 



Leptodesma aviforme, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



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-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 gently convex below, very 

 convex above, and moderately gibbous on the umbo. Right valve 

 unknown. Hinge-line straight; length a little more than the 

 greatest length of the valve. Beak sub-anterior, directed forward, 

 acute, not rising much above the hinge. Umbonal angle acute. 

 Anterior end narrow, elongate, very acute, limited by a shallow 

 byssal depression. Wing large, joining the body near the base ; 

 margin broadly and symmetrically concave ; extremity produced 

 into a spinif orm extension reaching beyond the posterior extent of 

 the shell. 



[Sen. Doc. No. 38.J 49 



