Lamellibkanchiata. 391 



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 extremity, undefined ; margin straight or slightly 

 concave, very oblique ; extremity slightly produced, angular. 



Test marked by regular, even, concentric striae, which, when well 

 preserved, are elevated and lamellose, and are crowded and fascicu- 

 late on the anterior end of the shell. 



A nearly entire left valve has a greatest length of 45 mm., height 

 20 mm., and hinge-line 26 mm. A similar right valve has a length 

 of 37 mm., height 18 mm., and hinge-line 22 mm. 



This species is very similar to L. Lichas, but differs in the greater 

 convexity of body, undefined byssal depression, and narrower wing. 



Formation and locality. In the middle portion of the Chemung 

 group at Mansfield, Tioga county, Pa. 



Leptodesma Lichas. 



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

 PI. 21, figs. 35-39. Jan., 1883. 



Shell of medium size, sub-rhomboidal ; body sub-cylindrical, very 

 oblique; length nearly twice the height; ante-byssal margin sub- 

 truncate, extending below to the broad, gently curving base; 

 posterior margin abruptly recurved. Valves very convex above, 

 gently convex toward the posterior extremity, making the body 

 somewhat cylindrical. Eight valve probably a little shorter than 

 the left, as inferred from the separated valves. Hinge-line straight, 

 less than the length of the shell. Beaks acute, situated at the 

 anterior third of the hinge-line, directed forward, prominent, arch- 

 ing over the cardinal line. Umbo abruptly gibbous, subtending an 

 acute angle. Anterior end large, acute and nasute at the extremity. 

 The byssal sinus makes only a slight depression in the margin. 

 Wing narrow-triangular, joining the body near the posterior ex- 

 tremity, not distinctly limited ; margin straight and oblique below, 

 recurving just beneath the hinge-line, and forming a short mucro- 

 nate extension. 



Test thin, marked by fine concentric striae which are irregularly 

 crowded into fascicles, producing an undulated aspect to the surface. 



