Shell. Irregular, inequivalve, inequilateral, roughly foliaceous, the left or inferior valve adhering, larger and deeper than the other, its summit prolonged with age into a sort of heel; the right or superior valve more or less operculiform; hinge oral, toothless; ligament subinterior, short, inserted in an oblong cardinal cavity increasing with the summit; muscular impression single and subcentral. Found in all seas near the mouths of rivers. Defrance enumerates one hundred and twenty species. Lamarck thirty-three fossil, forty-nine living.

2. Genus Gryphæa. Pl. [IX].

Animal. Unknown.

Shell. More finely lamellated than in the Ostrea, free, or slightly adhering, subequilateral, very inequivalve; the inferior valve very concave, with a summit more or less recurved in a hook; the superior opiculiform and much smaller; hinge toothless; ligament inserted in an elongated and arcuated cavity; a single muscular impression. Habitation unknown. One species.

3. Genus Vulsella. Pl. [IX].

Animal. Body elongated and compressed; mantle much prolonged posteriorly, and edged with two rows of very close tubercular papillæ; a moderately large abdominal foot, proboscidiform, canaliculated, without byssus; a very large transversal mouth with well-developed triangular labial appendages; branchiæ narrow, very long, and united in nearly all their extent.

Shell. Subnacred, irregular, flat, elongated, subequivalve, inequilateral, with summits anterior, distant and flexed inferiorly; hinge oral and toothless; ligament undivided, thick, inserted in a round pit excavated in a projecting apophysis upon each valve; a moderately large subcentral muscular impression, and two very small ones altogether anterior. Inhabits the Indian and Australasian seas. Six living species. One fossil.