358 Thirty-fifth Beport on the State Museum. 



Leptodesma potens. 



Leptodesma potens, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 

 PI. 21, figs. 21, 30 ; pi. 22, figs. 11, 12, 19, 21. Jan., 1883. 



Shell large, sub-rhomboidal ; body ovate, oblique, broad below ; 

 length more than one- third greater than the height; ante-byssal 

 margin sub-truncate, erect, oblique below ; basal margin broadly 

 curving; posterior margin broad, extended and abruptly recurving 

 at the upper extremity. Left valve convex, gibbous above ; the 

 greatest convexity at one-third the height of the shell from the 

 beak. Eight valve almost equally convex, more abruptly gibbous on 

 the umbo, and comparatively shorter and wider. Hinge-line straigh t, 

 about equal to the length of the shell. Beak sub-anterior, obtuse, 

 erect, prominent, arching over the hinge. Umbonal region gibbous, 

 very prominent, limited anteriorly by a shallow depression extend- 

 ing from a point anterior to the beak to the margin at one-third the 

 height of the shell below the hinge-line ; posteriorly limited by the 

 abrupt post-cardinal slope. Umbonal angle acute. Anterior ex- 

 tremity extended and angular. Wing large, triangular, joining the 

 body near the posterior extremity, distinctly limited ; margin broadly 

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

 which is unknown). 



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 equi-distant lamellae. Casts of the in- 

 terior show the concentric striae in a subdued condition. 



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

 to base, of 62 mm., 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 36 mm., height 28 mm., and hinge- 

 line, to base of spine, 35 mm. 



This species more nearly resembles L. robustum than any of the 

 forms here described ; but it is larger, the wing proportionally larger 

 and distinctly limited, and the sinus of 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 Vemeuili, while L. robustum is found only in the middle of 

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

 the right valves are very unlike. 



