Species orbicular, and not plaited; rugged, with undulated, imbricated scales; one valve flat and the other convex; variable in size; outside greenish brown, inside pearly white, sometimes with a bluish tinge. _O. Virginica._ The Virginian Oyster. Species longitudinal and not plaited. _O. imbricata._ The imbricated Oyster. Species orbicular and plaited. _O. crista-galli._ The Cock’s-comb Oyster. Species strongly plaited longitudinally. 3. Vulsella. The Tweezers. Six species. This genus has several characteristics which distinguish it from the Ostrea; they are free; the valves and the apices are nearly equal, with a projecting callosity on each valve, depressed underneath, and obliquely arched for the reception of the ligament. Shell sub-nacreous, sub-regular, sub-equivalve, inequilateral; upper valve finely granulated, or striated longitudinally from the apex to the margin; summits anterior, distant, recurved below; hinge without teeth; ligament undivided, thick, inserted in a rounded cavity, made in a slightly projecting callosity on each valve; muscular impression sub-central, rather large, and two very small ones entirely anterior. _V. lingulata._ The tongue-like Vulsella. Pl. 15, fig. 6. Elongated, depressed, transversely striated; pale yellowish brown, with longitudinal darker stripes. 4. Placuna. The Chinese Window Shell. Three species. 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.