Fam. 4. Pholadomyidae.—Mantle with fourth aperture; siphons very long, completely united, naked, incompletely retractile; foot small, with posterior appendage. Pholadomya.
Fam. 5. Arcomyidae.—Extinct; Secondary and Tertiary. Arcomya. Goniomya.
Fam. 6. Pholadellidae.—Extinct; Palaeozoic. Pholadella. Phytimya. Allorisma.
Fam. 7. Pleuromyidae.—Extinct; Secondary. Pleuromya. Gresslya. Ceromya.
Fam. 8. Pandoridae.—Shell thin, inequivalve, free; ligament internal; siphons very short. Pandora; British. Coelodon. Clidiophora.
Fam. 9. Myochamidae.—Shell very inequivalve, solid, with a pallial sinus; siphons short; foot small. Myochama. Myodora.
Fam. 10. Chamostraeidae.—A fourth pallial aperture present; pedal aperture small; siphons very short and separate; shell fixed by the right valve, irregular. Chamostraea.
Fam. 11. Clavagellidae.—Pedal aperture very small, foot rudimentary; valves continued backwards into a calcareous tube secreted by the siphons. Clavagella. Brechites (Aspergillum).
Fam. 12. Lyonsiidae.—Foot byssiferous; siphons short, invertible. Lyonsia; British. Entodesma. Mytilimeria.
Fam. 13. Verticordiidae.—Siphons short, gills papillose; foot small; shell globular. Many species abyssal. Verlicordia. Euciroa. Lyonsiella. Halicardia.