LAMELLIBRANCHIA TA. 
199 
Test thin, marked by fine concentric lines of growth which are irregularly 
crowded into small fascicles, producing a slightly undulated appearance. 
The striae are strong and regular over the wing. 
Interior unknown. 
The left valve described has a length of 41 mm., height 27 mm., hinge-line 
about 38 mm. 
This species bears some general resemblance to L. potens and L. robustum, but 
the body is narrower, less oblique and much more gibbous, the beak more 
elevated, and the posterior extremity of the body less expanded. The wing 
is also much larger than in L. robustum. It is very closely allied to L. umbonatum 
var. depressum, PI. xxii, fig. 14., but differs in its smaller mucronate anterior 
end; the body is more convex and narrower below, and the extremity of the 
wing more produced. 
Formation and locality. In a calcareous band of the upper part of the 
Chemung group, Twenty-mile creek, Chautauqua county, N. Y. 
Leptodesma umbonatum, var. depressum. 
PLATE XXII, FIG. 14; AND PLATE XC, FIG. 10. 
Leptodesma umbonatum, var. depressum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 22, 
fig. 14. Jan., 1883. 
Shell of medium size, rhomboidal; body very oblique-ovate; length once and 
a half greater than the height; ante-byssal margin, below the acute extrem¬ 
ity, regularly curving into the ventral margin; posterior extremity produced, 
with the margin gently recurving into the wing. 
Left valve convex below, gibbous above. Right valve smaller, a little less 
convex, abruptly gibbous on the umbo. 
Hinge-line straight, somewhat less than the greatest length of the shell. 
Beaks sub-anterior, obtuse, directed slightly forward, very prominent, 
arching over the hinge-line. 
Anterior end short, large, limited by a broad, undefined depression; 
extremity produced, acute. Wing large, triangular, not distinctly defined, 
