Shell. Subnacreous, irregular, subequivalve, inequilateral, generally much auriculated anteriorly, and prolonged posteriorly into the body, so as to present some resemblance to a hammer; summits entirely anterior; between them and the inferior auricle, an oblique slope for the passage of the byssus; hinge linear, very long, buccal, toothless; ligament simple, triangular, inserted in a conical oblique pit, partly external; a moderately large subcentral muscular impression. Inhabits the Australian seas. Six species.

4. Genus Avicula. Pl. [IX].

Animal. Body much compressed; mantle cleft throughout its circumference, except along the back, and garnished at its free edge with a double row of very short tentacular cirri; foot small, canaliculated; a byssus; mouth surrounded with fringed lips, besides two pair of labial appendages, a large contractile muscle, nearly posterior.

Shell. Foliaceous or not; always nacred, subequivalve, of a subregular form, but somewhat variable; valves oblique, the left one with a little notch, through which the byssus passes; hinge linear, toothless, or with two small rudimentary teeth; ligament more or less exterior, placed in a narrow groove; one very large posterior muscular impression and one very small anterior. Inhabits the British and American coasts. Nineteen species.

5. Genus Meleagrina. Pl. [IX].

Animal. As above.

Shell. Subequivalve, rounded, squamous, a sinus posteriorly for the passage of the byssus, at which place the left valve is notched and narrow; hinge linear and destitute of teeth; ligament marginal, elongated, partly exterior, and dilated in the centre. Inhabits the Indian Ocean. Two species, of which the first is the pearl-oyster.