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.
5. Anomia. The Antique Lamp. Six species.
When Linnæus formed this genus and named it Anomia, he probably did so from its having no determinate character. Its common name was given it by the fancied resemblance of some of its species to an antique lamp. Like the oyster, they seldom leave their place; they are always affixed to marine bodies by an osseous operculum, formed by the thick extremity of the animal’s muscle. The lower valve is perforated and smaller, conforming to the shape of the substance to which it is affixed.
Shell adhering, irregular, inequivalve, inequilateral, ostraceous; inferior valve rather more flat than the superior, divided at the summit into two sloping branches, whose approaching together forms a large oval hole, through which protrudes a muscle, the extremity of which becomes ossified and adheres to extraneous bodies; one sub-central muscular impression divided into three.
Anomia ephippium.
A. patellaris.