Fam. 3. Astartidae.—Shell concentrically striated; foot elongate, without byssus. Astarte; British. Woodia. Opis; Secondary. Prosocoelus; Devonian.
Fam. 4. Crassatellidae.—Shell thick, with concentric striae, ligament external; foot short. Crassatella. Cuna.
Fam. 5. Carditidae.—Shell thick, with radiating costae; foot carinated, often byssiferous. Cardita. Thecalia. Milneria. Venericardia.
Fam. 6. Condylocardiidae.—Like Carditidae, but with an external ligament. Condylocardia. Carditella. Carditopsis.
Fam. 7. Cyprinidae.—Mantle open in front, with two pallial sutures; external gill-plates smaller than the internal. Cyprina; British. Cypricardia. Pleurophorus; Devonian to Trias. Anisocardia; Jurassic to Tertiary. Veniella; Cretaceous to Tertiary.
Fam. 8. Isocardiidae.—Mantle largely closed, pedal orifice small; gill-plates of equal size; shell globular, with prominent and coiled umbones. Isocardia; British.
Fam. 9. Callocardiidae.—Siphons present; external gill-plate smaller than the internal; umbones not prominent. Callocardia; abyssal.
Fam. 10. Lucinidae.—Labial palps very small; gills without an external plate. Lucina; British. Montacuta; British. Cryptodon.
Fam. 11. Corbidae.—Shell thick, with denticulated borders; anal aperture with valve but no siphon; foot elongated and pointed. Corbis. Gonodon; Trias and Jurassic. Mutiella; Upper Cretaceous.