LAMELLIBRANCHIA TA. 
185 
Formation and localities. In the Chemung group at Chemung Narrows, N. Y., 
and Tioga county, Pa. 
Leptodesma Becki. 
PLATE XXII, FIGS. 3-5. 
Leptodesma Becki, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 22, figs. 3-5 (and 
figs. 6 and 7 in error). Jan., 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 gibbous except in the lower part. Right valve less convex. 
Hinge-line straight, less than the length of the shell, and greater than 
the height. 
Beaks at about the anterior third of the hinge-line, obtuse, directed 
forward and arching over the hinge. Umbonal region gibbous, scarcely 
defined anteriorly, but well-marked posteriorly by the abrupt 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 by concentric striae, which are crowded and somewhat 
lamellose in front, and closely and evenly arranged on the wing. The hinge¬ 
line has a single narrow groove. 
A left valve has a length of 28 mm., height 17 mm., hinge-line about 
22 mm. A detached right valve has a length of 29 mm., height 20 mm., 
hinge-line 30 mm. 
This species resembles L. robustum in general expression and in the sinus of 
the wing, but the body is narrower, more regularly convex, the base less expanded, 
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