LAMELLIBRANCHIA TA. 
231 
Anterior end short, acute at the extremity, defined by a slight bend in the 
striae indicating the byssal sinus. Wing small, narrow-triangular, joining 
the body near the posterior end; margin straight, very oblique; extremity 
angular. 
Test marked by strong concentric striae, which at equal intervals are raised 
into sharp lamellae, and are more crowded in front. 
Interior unknown. 
A left valve has a length of 47 mm., height 25 mm., and hinge-line 27 mm. 
This species is remarkable for its extreme obliquity of body. In its general 
features it somewhat resembles L. arciforme , but differs in the less angular 
gibbosity of the body, larger wing, more extended anterior end, and in the 
surface markings. 
Formation and locality. In the upper part of the Chemung group at Mansfield, 
Tioga county, Pa. 
Leptodesma propinquum, n. sp. 
PLATE XCI, FIGS. 16,17. 
Shell of medium size, rhomboidal; body narrow, sub-cylindrical, very oblique, 
making an angle of about 30° with the hinge-line; length more than twice 
the height; anterior margin oblique; ventral margin nearly straight on the 
anterior half, and broadly curving over the lower half ; posterior margin 
abruptly curved. 
Yalves gibbous, nearly equal in size and convexity. 
Hinge-line straight, longer than the height of the valve. 
Beaks at about the anterior third of the hinge-line, directed forward, very 
prominent, arching over the cardinal line. Umbonal region gibbous, limited 
posteriorly by the vertical descent of that side, scarcely defined on the 
anterior. The umbo and post-cardinal slope are obtusely sub-angular for 
about two-thirds of the length of the body. Umbonal angle acute. 
Anterior end not defined (the byssal depression being obsolete); extremity 
acute. Wing small, narrow-triangular, joining the body near the posterior 
