LAMELLIBRANCHIATA. 185 



Formativn and localities. In the Chemung group at Chemung Narrows, N. Y., 

 and Tioga county, Pa. 



Leptodesma Becki. 



PLATE XXII, FIGS. .-J-o. 



Leptodesma Becki, Hall. Pal. N. V.. vol. v. pt. 1. Plates and Explanations: PI. 22, Hgfi. 3-5 (and 



tiif.s. (i anil 7 in rn'or). .Ian., 1883. 



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

 left valve ; length more than one-third greater than the height ; anterior 

 margin oblique, extending in nearly a direct line into the broadly rounded 

 basal margin ; post-basal margin produced, not abruptly recurved into the 

 wing. 



Left valve gibljous except in the lower part. Right valve less convex. 



Hinge-line straight, less than the length of the shell, and greater than 

 tlie height. 



Beaks at about the anterior third of the hinge-line, obtuse, directed 

 forward and arching over the hinge. Umbonal region giblaous, scarcely 

 defined anteriorly, Init well-marked posteriorly by the aljrupt slope of the 

 side. Umbonal angle acute. 



Anterior end truncate, acute at the extremity. Wing of medium size, 

 not distinctly limited, joining the body nearly at the posterior extremity ; 

 margin concave ; extremity produced into a short spine. The right valve 

 is proportionally higher, the body less distinctly defined, and the wing larger. 



Test thin, marked b}' concentric stria?, whicli are crowded and somewhat 

 lamellose in front, and closely and evenly arranged on the wing. The liinge- 

 line has a sintjle narrow sfroove. 



A left valve has a length of 28 mm., height 17 nun., hinge-line about 

 22 mm. A detached right valve has a length of 2'.> mm., height 20 mm., 

 hinge-line 30 mm. 



This species resembles L. rohustum in general expression and in .the sinus of 

 the wing, but the body is narrower, more regularly convex, the base less expanded, 

 24 



