Lamellibranchiata. 375 



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. 



Letpodesma trukcatum. 



Leptodesma iruncatum, Hall, Pal. N. Y„, vol. v, pt. 1. 



Shell above the medium size, sub-rhomboidal ; body ovate-elongate, 

 oblique to the hinge-line at an angle of about 35° ; length two-thirds 

 greater than the height; ante-byssal margin oblique and often 

 abruptly truncate, curving into the sinus ; broadly rounded along 

 the ventral margin, and more rapidly curving on the post-basal 

 margin. Left valve gently convex below, becoming more convex 

 above, and somewhat gibbous on the umbo. Eight valve less con- 

 vex, flat in the lower part. Hinge-line straight, nearly as long as the 

 length of the shell. Beak sub-anterior, directed forward ; in the 

 left valve somewhat prominent ; in the right valve not elevated above 

 the hinge-line. Umbonal angle acute. Anterior end obtuse, 

 truncate, abruptly angular at the cardinal margin, limited by a 

 slightly oblique byssal depression. Wing large, triangular, join- 

 ing the body at the posterior extremity ; margin sloping gently 

 forward for two-thirds of the width, then abruptly recurved and 

 extended in an acute or spiniform process. In the right valve the 

 wing is less defined, and the anterior end is more strongly truncate. 



Test thin, marked by sharp, elevated, distant, concentric striae, 

 with intermediate finer lines of growth. The stronger striae are 

 regular upon the wing and posterior part of the shell, fasciculate on 

 the anterior side, and stronger and lamellose on the anterior ex- 

 tremity. 



A left valve has a length of 38 mm., height 22 mm., and hinge- 

 line 37 mm. 



This species may be known by its truncate anterior end. The form 

 of the body is very similar to L. extenuatum, but it is somewhat more 

 convex and a little wider; the wing margin is less concave and the 

 extremity less extended. The right valves of these two species are 

 conspicuously different. 



Formation and locality. In the central portion of the Chemung 

 group, Mansfield, Tioga county, Pa. 



