This section comprises the families Ostreidae, Aviculidae, Mytilidae, Arcadae, Trigoniadae, and Unionidae.
The Ostreidae (including the Oysters, Scallops, Anomiae, Thorny Oysters, etc.) are all marine, and are monomyary. The Aviculidae, or Pearl-oysters, are likewise marine, but are dimyary. The Mytilidae (Mussels, Horse-mussels, etc.) are partially marine and partially fresh-water forms, and have a very small anterior adductor. The Arcadae (Ark-shells, etc.) arc exclusively marine, as are the nearly allied Trigoniadae. Lastly, the Unionidae (Fresh-water Mussels) are exclusively confined to rivers and lakes.
Animal with respiratory siphons; mantle-lobes more or less united.
Two subdivisions, of little classificatory or anatomical value, are comprised in this section. In the first the siphons are short, and the pallial line is simple (Integropallialia); as is seen in the families Chamidae, Hippuritidae, Tridacnidae, Cardiadae, Lucinidae, Cycladidae, and Cyprinidae.
The second subdivision (Sinupallialia) is distinguished by the possession of long respiratory siphons, and a sinuated pallial line, and it comprises the families Veneridae, Maclridae, Tellinidae, Solenidae, Myacidae, Anatinidae, Gastrochaenidae, and Pholadidae.
The Chamidae (Thorny Clams) are fixed to foreign bodies by the substance of either valve indifferently, and are all inhabitants of the sea. The extraordinary extinct group of the Hippuritidae, from the fossils associated with them, are known to have been also marine; and they are often found in great beds like Oysters, attached to one another and to foreign objects by the beak of the right valve. The Tridacnidae (Giant Clams) have a similar habitat, and the shell may attain a weight of five hundred pounds. The Cardiadae (Cockles) and Lucinidae are also marine, as are the Cyprinidae; but the Cycladidae are fresh-water and brackish-water forms. The Veneridae (Clams) are amongst the most beautiful of the Bivalves, and are found in all seas, attaining their maximum in warm regions. The Mactridae (Trough-shells) and Tellinidae are mostly marine, though also found in brackish waters ; and the Solenidae (Razor-shells), Myacidae, and Anatinidae are essentially marine, though some of the Myacidae extend their range for a considerable distance above the mouths of rivers. The Gastrochaenidae are all natives of the sea, and have a burrowing habit, boring holes for habitation in rocks, or living in the mud. Lastly, the Pholadidae (Piddocks and Ship-worms) bore holes in stone or wood, in which they live, and are all marine in habit. The Ship-worms (Teredo) have long worm-like bodies, and do an immense amount of harm by honeycombing with their burrows the sides of ships, or other wooden structures immersed in the sea.