Fam. 21. Rangiidae.—Two short siphons, shell with prominent umbones and internal ligament. Rangia; brackish water, Florida.
Fam. 22. Cardiniidae.—Shell elongated, inequilateral. Extinct. Cardinia; Trias and Jurassic. Anthracosia; Carboniferous and Permian. Anoplophora; Trias. Pachycardia; Trias.
Fam. 23. Megalodontidae.—Shell inequilateral, thick; posterior adductor impression on a myophorous apophysis. Extinct. Megalodon; Devonian to Jurassic. Pachyrisma; Trias and Jurassic. Durga; Jurassic. Dicerocardium; Jurassic.
Fam. 24. Unionidae.—Shell equilateral; mantle with a single pallial suture and no siphons; freshwater; larva a glochidium. Unio; British. Anodonta; British. Pseudodon. Quadrula. Arconaia. Monocondylea. Solenaia. Mycetopus.
Fam. 25. Mutelidae.—Differs from Unionidae in having two pallial sutures; freshwater. Muleta. Pliodon. Spatha. Iridina. Hyria. Castalia. Aplodon. Plagiodon.
Fam. 26. Aetheriidae.—Shell irregular, generally fixed in the adult; foot absent; freshwater. Aetheria. Mulleria. Bartlettia.
Sub-order III.—Tellinacea.
Mantle not extensively closed; two pallial sutures and two well-developed siphons. Gills smooth. Foot compressed and elongated. Labial palps very large. Dimyarian; pallial line with a deep sinus.
Fam. 1. Tellinidae.—External gill-plate directed upwards; siphons separate and elongated; foot with byssus; palps very large; ligament external. Tellina; British. Gastrana; British. Capsa. Macoma.
Fam. 2. Scrobiculariidae.—External gill-plates directed upwards; siphons separate and excessively long; foot without byssus. Scrobicularia; estuarine; British. Syndosmya; British. Cumingia.