LA MELLIBRAN CHI A TA. 
217 
Leptodesma nereus, n. sp. 
PLATE XC, FIGS. 31-33. 
Shell of medium size, sub-rhomboidal; body broadly ovate, narrowing rapidly 
to the anterior end, and oblique to the hinge-line at an angle of 45°; length 
less than one-third greater than the height; ante-byssal margin gently curved, 
extending into a wide sinus, broadly rounded along the base, curving regularly 
on the post-basal side and abruptly recurved in the upper part. 
Left valve gently convex below, more convex above, and moderately 
gibbous on the umbo. Right valve smaller, more depressed below, nearly 
equally convex toward the umbo. 
Hinge-line straight, less than the length of the valve. 
Beaks sub-anterior, acute, directed forward, rising slightly above the 
hinge-line. Umbonal region gibbous in the left valve, narrower but equally 
gibbous in the right valve. Umbonal angle acute. 
Anterior end small, abruptly curving into the byssal depression ; extremity 
acute, not prolonged. Wing broad, extending along the body about four- 
fifths of its length from the beak; margin very oblique in the lower part 
and moderately concave above the centre, then turning abruptly backward 
just below the cardinal line, and produced into a short spiniform extension. 
In the right valve the wing is less defined and less concave on the margin. 
Test thin, marked by concentric striae, which are fasciculate on the body 
and anterior part, and more elevated, distant and sharp upon the wing. 
Muscular impression large, sub-circular, situated on the posterior slope 
below the middle of the length. The pallial line extends from the lower 
part of the muscular impression nearly parallel to the anterior and basal 
margin. The hinge of the left valve has a strong groove extending its entire 
length; the right valve has an oblique posterior fold. 
One specimen has a length of 35 mm., height 26 min., hinge-line 31 mm. 
A similar example has a length of 34 mm., height 24 mm., and hinge-line 
28 mm. A right valve has a greatest length of 31 mm., height 20 mm., and 
hinge-line 25 mm. 
28 
