LAMELLIBRANCHIA TA. 
207 
Anterior end short in the direction of the hinge-line, separated from the 
body by a well-marked byssal depression extending for half the height of the 
valve; extremity abruptly angular. Wing large, joining the body below the 
middle of its length, obscurely defined from the body of the valve; margin 
moderately oblique, nearly straight, or slightly convex below, with a very 
gentle concavity just beneath the cardinal line. 
Test thin; casts of the interior obscurely marked by fine striae of growth 
which are sometimes fasciculate upon the body of the shell. 
Interior unknown. 
An individual of medium size has a length of 32 mm., height 27 mm., and 
hinge-line 23 mm. A smaller example has a length of 25 mm., height 20 
mm., and hinge-line 18 mm. 
This species is distinguished from L. Maclurii by its shorter form, less 
parallel sides of the body, larger wing, which is not extended on the cardinal 
margin, and the body is not angular along its entire length. 
The specimen, PI. xxv, fig. 10, has been flattened by pressure and is referred 
with doubt to this species. It has the same general aspect, but is proportionally 
wider below the middle, and more broadly rounded on the post-basal margin, 
which peculiarities may have been produced by compression. 
Formation and locality. In a coarse sandstone of the Upper Chemung group, 
on the road from Olean, N. Y., to Smethport, Pa. 
Section c, rostrata. 
Leptodesma extenuattjm. 
PLATE XXXI, FIG. 23 ; and PLATE XC, FIGS. 17, 18. 
Leptodesma extenuatum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PL 22, fig. 23. 
Jan., 1883. 
Shell of medium size, elongate, semi-ovate; body narrowly ovate and oblique 
to the hinge-line at an angle of about 30°; length nearly twice the height; 
ante-byssal margin very oblique, scarcely depressed at the sinus, thence nearly 
straight to the broadly curved base; posterior margin abruptly recurved. 
