LAMELLIBRANCHIA TA. 
209 
Leptodesma Hector, n. sp. 
PLATE XC, FIGS. 19, 20. 
Shell of medium size, sub-rhomboidal; body narrowly elongate-ovate, oblique 
at an angle of about 30° with the cardinal line; length a little more than 
once and a half the height; ante-byssal margin oblique, curving into a broad, 
shallow sinus; ventral margin broadly curved; posterior margin abruptly 
recurved. 
Left valve moderately convex below, narrowly gibbous above. Right 
valve depressed below, gently convex above. 
Hinge-line straight; length equal to about four-fifths of the length of the 
valve. 
Beaks sub-anterior, directed forward, acute, slightly elevated above the 
hinge. Umbonal angle very acute. 
Anterior end narrow, small, produced into an acute extension, defined by 
a shallow byssal depression. Wing large, joining the body at three-fourths 
or more of its length from the beak; margin very oblique below, deeply 
sinuate just beneath the cardinal line, turning abruptly backward and 
terminating in a spiniform extension. The wing of the right valve is 
scarcely defined, and the shell has a somewhat broadly spatulate aspect. 
Test thin, marked by fine, close, concentric striae, and, as usually seen, 
the surface shows nearly equally distant lamellose striae on the body and 
wing, which are crowded on the anterior side and extension. In the right 
valve these characters are more subdued on the body than on the wing. In 
ordinary conditions, the specimens mostly exhibit only obscure indications 
of the striation of the shell. 
Muscular impression in the right valve large, sub-circular, placed below 
the middle of the length of the posterior slope. 
A left valve, of the usual size, has a length of 35 mm., height 20 mm., 
hinge-line 27 mm. A small specimen has a length of 20 mm., height 12 
mm., and hinge-line 16 mm. Young shells of this species are more gibbous 
than the older examples. 
27 
