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 tbe 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 X( . FIU 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. (Jmbonal 

 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 stria-, 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, ami elevated on the wing. 



Interior unknow n. 



A left valve has a length of -'17 mm., height 20 mm., hinge-line 32 mm. 



This species differs from L. extenuatum in its broader body, shorter hinge-line, 



ami narrower win-: the strise bend more abruptly forward along the post- 

 cardinal slope, and the win- margin is more deeply concave. 



Formation and localities. In -hales of the Chemung -roup at Kelly's creek 

 and Mansfield, Tioga countj . Pa. 



