Lamellibranchiata. 371 



Leptodesma Mentor. 



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 Unpublished. 



Shell of medium size, sub-rhomboidal ; body elongate-ovate, narrow 

 at the anterior end, oblique, making an angle of about 40° with the 

 hinge-line ; length one-third greater than the height ; ante-byssal 

 margin oblique, curving into the gently depressed sinus; basal 

 margin broadly curving into the rounded posterior extremity. Left 

 valve moderately convex in the lower part, becoming convex in the 

 middle and gibbous above. Right valve unknown. Hinge-line 

 straight; length about equal to the height of the valve. Beak sub- 

 anterior, directed forward, acute, but little elevated above the hinge- 

 line. Umbonal region abruptly, or sub-angularly gibbous, narrow, 

 subtending an acute angle. Anterior end small, narrow, limited by 

 an oblique byssal depression which reaches about half way down 

 the body of the shell ; extremity pointed. Wing joining the body 

 near the post-extremity of the shell, denned by a shallow groove ; 

 margin oblique, straight, apparently slightly concave just below the 

 cardinal line. 



Test thin, leaving upon the cast the marks of the striae of growth 

 which have been somewhat fascicled upon the body, giving it an 

 obscurely undulated appearance. 



A specimen of this species has a length of 35 mm., height 24 mm., 

 and hinge-line nearly 24 mm. 



This species resembles L. Or odes, but differs in its greater obliquity; 

 the body is more attenuate toward the anterior, and the wing is dis- 

 tinctly limited. This one, and several other species, have shown the 

 presence of an alar furrow and obscure fold, similar to Ptychopteria. 



Formation and locality. In a coarse sandstone of the Upper Che- 

 mung group, south of Smethport, McKean county, Pa. 



Leptodesma Orodes. 



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

 PI. 25, figs. 6, 9 (10 ?). Jan. , 1883. 



Shell of medium size, sub-rhomboidal ; body ovate, oblique to the 

 hinge-line at an angle of about 55°; length one-fifth greater than 

 the height ; ante-byssal margin a little oblique, nearly vertical above, 

 curving into a long, undefined sinus ; ventral margin rounded into 

 the broad posterior curve. Left valve moderately convex in the 

 lower part, increasing in convexity to the middle of the length, 



