This genus received its common name from the thin, transparent nature of
the valves of the shell, particularly of the species _placenta_, which
by the ingenious Chinese are often polished and used as a substitute for
window-glass.
The hinge of the shells of this genus is so peculiar as to make it
perfectly distinct; entirely interior, fastened by a ligament shaped
like a V on one of the valves.
Shell free, sub-irregular, very thin, almost entirely transparent, flat,
sub-equivalve, sub-equilateral, slightly auricled; hinge entirely
internal, formed on the superior less valve by two elongated, unequal,
oblique ribs converging at the summit, to the interior side of which a
ligament like the letter V is inserted in two equally converging, rather
deep cavities of the lower valve, which is more convex; one rather
small, sub-central muscular impression.
_P. placenta._ The glassy Placuna. Pl. 16, fig. 3.
Sub-orbicular, flat, white, and transparent; finely striated
longitudinally, slightly decussated.