374 Thirty-fifth Report ok the State Museum. 



body and wing, which are crowded on the anterior side and exten- 

 sion. In the right valve these characters are more subdued on the 

 body than on the wing. In ordinary conditions, the specimens 

 mostly exhibit only obscure indications of the striation of the shell. 

 Muscular impression in the right valve large, sub-circular, placed 

 below the middle of the length of the posterior slope. 



A left valve, of the usual size, has a length of 35 mm., height 20 

 mm., hinge-line 27 mm. A small specimen has a length of 20 mm., 

 height 12 mm., and hinge-line 16 mm. Young shells of this species 

 are more gibbous than the older examples. 



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 pro- 

 longed. 



Formation and locality. In the upper shales of the Chemung group 

 near Canton, Bradford county, Pa. 



Leptodesma Clitus. 

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 Unpublished. 



Shell of medium size, narrowly sub-rhomboid al ; 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. Bight valve unknown. Hinge- 

 line straight, less than the greatest length of the shell. Beak sub- 

 anterior, directed forward, moderately prominent. TJmbonal region 

 gibbous, subtending an acute angle. Anterior end narrow, pro- 

 longed into a rostrate extension, defined by a strong byssal depres- 

 sion. Wing narrow-triangular, joining the body near the posterior 

 end, defined only by the abrupt recurving of the strias ; 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. 



A left valve has a length of 37 mm., height 20 mm., hinge-line 

 32 mm. 



