LAMELLIBRANCHIATA. 189 



sinuate ; extremity produced into a strong spine (the extent of whicli is 

 unlinown). 



Surface marked by strong concentric lines of growth, which are crowded 

 into small fascicles at irregular intervals on the body of the shell, and become 

 lamellose expansions on the anterior. On the wing they appear as sharp 

 equidistant lamella'. Casts of the interior show the concentric striae in a 

 sut)dned condition. 



Interior unknown. 



The largest left valve observed has a length, measured from beak to base, 

 of 02 nun., height 46 mm., hinge-line, exclusive of the spine, 58 mm. A 

 smaller specimen has a length of 57 mm., height 39 mm., hinge-line, to base 

 of spine, 58 mm. A right valve referred to this species has a length of M6 

 nun., height 28 mm., and hinge-line, to base of spine, 35 mm. 



This species more nearly resembles L. rohustum than anv of the forms here 

 described ; but it is larger, the wing proportionally larger and distinctly limited, 

 and the siiuis ol" the wing more central. The right valve is also more convex 

 and the wing more deeply sinuate. It occurs in the upper members of the 

 Chemung group, associated with Spirifera Venceuili, while L. robustum is found 

 only in the middle of the series. 



It is a larger and more robust shell than L. Mortoni, and the right valves 

 are very unlike. 



Formation and localities. In tlie Upper Chemung group at Olean, Portville, 

 Cassadaga lake, and in loose specimens collected near Panama, N. Y. 



Leptodesma potens, var. juvens. 



PLATE XXU, FIG. 16. 



Leptodesma potens. mr. jmvn.'i. Hall. Pal. N. Y., vol. v, jit. 1. Plate.'^ and Rxiilanatioiis : PI. 22, tig-. 10. 



Jan., 18S3. 



Shell small, rhomboidal ; body ovate, oblique ; the greatest length more than 

 once and a half the height; anterior margin oblique, very broadly curving 

 below ; post-basal extremity rounded and abruptly recurved. 



Left valve convex below, vexy gibbous and ventricose in the middle and 

 above. Right valve unknown. 



