4. Genus Placuna. Pl. [IX].
Animal. Entirely unknown.
Shell. Free, subirregular, very fine, almost entirely translucid, flat, subequivalve, subequilateral, slightly auriculated; hinge altogether internal, formed upon the superior valve, which is the smaller, by two elongated, unequal, oblique, crests; converging to the summit, at the internal side of which a ligament is attached in the form of a V, a single, small, subcentral muscular impression. Inhabits the Indian seas. Two fossils in France. Three living species.
- Placuna sella.
- P. papyracea.
- P. placenta.
5. Genus Anomia. Pl. [IX].
Animal. Much compressed; edges of the mantle very fine; not adhering, and furnished exteriorly with a row of tentacular filaments; contractile muscle thick, divided into three parts, the largest of which passes partially across a slope of the inferior valve, and often contains a calcareous substance or small bone, adhering to marine bodies.
Shell. Adhering, irregular, inequivalve, inequilateral, ostraceous; inferior valve a little flatter than the superior, divided at the summit into two sloping branches, whose approximation forms a large oval hole; the superior valve, which is the larger, has an oval excavation under the summit; a subcentral muscular impression, divided into three parts. Inhabits the British coasts. Two beautiful species are found on the shores of Rhode Island and New Jersey. Nine species.
- Anomia patellaris.
- A. pyriformis.
- A. lens.
- A. cepa.
- A. electrica.
- A. membranacea.
- A. squamula.
- · · · · ·
- A. ephippium.*
- A. fornicata.*
6. Genus Crania. Pl. [IX].
Animal. See Anomia.