LAMELLIBRANCHIATA. 231 



Anterior end short, acute at the extremity, defined by a slight bend in the 

 strife 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 stria', which at equal intervals are raised 

 into sharp lamella?, 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. 



Valves 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 



