210 
PALAEONTOLOGY OF NEW YORK. 
This species resembles, in its general form, L. extenuatum , but the body is 
narrower at the beak, the wing less extended, and the greatest concavity of the 
margin is just below the cardinal line; while in that species it is near the 
middle; the anterior extremity is also less prolonged. 
Formation and locality. In the upper shales of the Chemung group near 
Canton, Bradford county, Pa. 
Leptodesma Clitus, n. sp. 
PLATE XC, FIG. 21. 
Shell of medium size, narrowly sub-rhomboidal; body elongate-ovate, oblique 
to the hinge-line at an angle of about 30°; length less than twice the height; 
ante-byssal margin very oblique ; ventral margin broadly rounded, somewhat 
abruptly recurved at the post-basal extremity. 
Left valve depressed-convex below, convex above, moderately gibbous on 
the umbo. Right valve unknown. 
Hinge-line straight, less than the greatest length of the shell. 
Beak sub-anterior, directed forward, moderately prominent. Umbonal 
region gibbous, subtending an acute angle. 
Anterior end narrow, prolonged into a rostrate extension, defined by a 
strong byssal depression. Wing narrow-triangular, joining the body near 
the posterior end, defined only by the abrupt recurving of the striae; margin 
deeply and abruptly concave, the deepest concavity about the middle; the 
extremity is an acute extension of the cardinal line. 
Test thin, marked by fine concentric striae, which are fasciculate upon the 
body, leaving the cast marked by gentle undulations, which are lamellose and 
elevated on the anterior side, and rostral extension, and elevated on the wing. 
Interior unknown. 
A left valve has a length of 37 mm., height 20 mm., hinge-line 32 mm. 
This species differs from L. extenuatum in its broader body, shorter hinge-line, 
and narrower wing; the striae bend more abruptly forward along the post¬ 
cardinal slope, and the wing margin is more deeply concave. 
Formation and localities. In shales of the Chemung group at Kelly’s creek 
and Mansfield, Tioga county, Pa. 
