Fam. 4. Radiolitidae.—Shell conical or biconvex, without canals in the external layer. Radiolites. Biradiolites.
Fam. 5. Hippuritidae.—Fixed valve long, cylindro-conical, with three longitudinal furrows which correspond internally to two pillars for support of the siphons. Hippurites. Arnaudia.
Sub-order VII.—Myacea.
Mantle closed to a considerable extent; siphons well developed; gills much folded and frequently prolonged into the branchial siphon. Foot compressed and generally byssiferous. Shell gaping, with a pallial sinus.
Fam. 1. Psammobiidae.—Siphons very long and quite separate; foot large; shell oval, elongated, ligament external. Psammobia; British. Sanguinolaria. Asaphis. Elizia. Solenotellina.
Fam. 2. Myidae.—Siphons united for the greater part of their length, and with a circlet of tentacles near their extremities; foot reduced; shell gaping; ligament internal. Mya; British. Sphenia; British. Tugonia. Platyodon. Cryptomya.
Fam. 3. Corbulidae.—Shell sub-trigonal, inequivalve; pallial sinus shallow; siphons short, united, completely retractile; foot large, pointed, often byssiferous. Corbulomya. Paramya. Erodona and Himella are fluviatile forms from South America.
Fam. 4. Lutrariidae.—Mantle extensively closed; a fourth pallial aperture behind the foot; siphons long and united; shell elongated, a spoon-shaped projection for the ligament on each valve. Lutraria; British. Tresus. Standella.
Fam. 5. Solenidae.—Elongated burrowing forms; foot cylindrical, powerful, without byssus; shell long, truncated and gaping at each end. Solenocurtus; British. Tagelus; estuarine. Ceratisolen; British. Cultellus; British. Siliqua. Solen; British. Ensis; British.
Fam. 6. Saxicavidae.—Mantle extensively closed, with a small pedal orifice; siphons long, united, covered by a chitinous sheath; gills prolonged into the branchial siphon; foot small; shell gaping. Saxicava; British. Glycimeris. Cyrtodaria.