Sub-order I.—Ostraeacea.
Monomyarian or with a very small anterior adductor. Mantle open; foot rather small; branchiae folded; shell inequivalve.
Fam. 1. Limidae.—Shell with auriculae. Foot digitiform, with byssus. Borders of mantle with long and numerous tentacles. Gills not united with mantle. Lima; members of this genus form a nest by means of the byssus, or swim by clapping the valves of the shell together. Limaea.
Fam. 2. Ostraeidae.—Foot much reduced and without byssus. Heart usually on the ventral side of the rectum. Gills fused to the mantle. Shell irregular, fixed in the young by the left and larger valve. Ostraea; foot absent in the adult; edible and cultivated; some species, as the British O. edulis, are hermaphrodite.
Fam. 3. Eligmidae.—Extinct; Jurassic.
Fam. 4. Pinnidae.—Shell elongated, truncated and gaping posteriorly. Dimyarian, with a very small anterior adductor. Foot with byssus. Pinna; British. Cyrtopinna. Aviculopinna; fossil, Carboniferous and Permian. Pinnigena; Jurassic and Cretaceous. Atrina; fossil and recent, from Carboniferous to present day.
Sub-order II.—Submytilacea.
Mantle only slightly closed; usually there is only a single suture. Siphons absent or very short. Gills smooth. Nearly always dimyarian. Shell equivalve, with an external ligament.
Fam. 1. Dreissensiidae.—Shell elongated; hinge without teeth; summits of valves with an internal septum. Siphons short. Dreissensia; lives in fresh water, but originated from the Caspian Sea; introduced into England about 1824.
Fam. 2. Modiolarcidae.—Foot with a plantar surface; the two branchial plates serve as incubatory pouches. Modiolarca.