Fam. 1. Arcidae.—Borders of the mantle bear compound pallial eyes. The labial palps are direct continuations of the lips. Hinge pliodont, that is to say, it has numerous teeth on either side of the umbones and the teeth are perpendicular to the edge. Arca; foot byssiferous; British. Pectunculus; foot without byssus; British. Scaphula; freshwater; India. Argina. Bathyarca. Barbatia. Senilia. Anadara. Adacnarca.
Fam. 2. Parallelodontidae.—Shell as in Arca, but the posterior hinge teeth elongated and parallel to the cardinal border. Cucullaea; recent and fossil from the Jurassic. All the other genera are fossil: Parallelodon; Devonian to Tertiary. Carbonaria; Carboniferous, &c.
Fam. 3. Limopsidae.—Shell orbicular, hinge curved, ligament longer transversely than antero-posteriorly; foot elongate, pointed anteriorly and posteriorly. Limopsis. Trinacria; Tertiary.
Fam. 4. Philobryidae.—Shell thin, very inequilateral, anterior part atrophied, umbones projecting. Philobrya.
Fam. 5. Cyrtodontidae.—Extinct; shell equivalve and inequilateral, short, convex. Cyrtodonta; Silurian and Devonian. Cypricardites, Silurian. Vanuxemia; Silurian.
Fam. 6. Trigoniidae.—Shell thick; foot elongated, pointed in front and behind, ventral border sharp; byssus absent. Trigonia; shell sub-triangular, umbones directed backwards. This genus was very abundant in the Secondary epoch, especially in Jurassic seas. There are six living species, all in Australian seas. Living specimens were first discovered in 1827. Schizodus; Permian. Myophoria; Trias.
Fam. 7. Lyrodesmidae.—Extinct; shell inequilateral, posterior side shorter; hinge short, teeth in form of a fan. Lyrodesma; Silurian.
Sub-order III.—Mytilacea.
Symmetrical, the anterior adductor small or absent. Heart gives off only an anterior aorta. Surface of gills smooth, gill-filaments all similar, with interlamellar junctions. Gonads generally extend into mantle and open at sides of kidneys. Foot linguiform and byssiferous.
Fam. 1. Mytilidae.—Shell inequilateral, anterior end short; hinge without teeth; ligament external. Mantle has a posterior suture. Cephalic eyes present. Mytilus; British. Modiola; British. Lithodomus. Modiolaria; British. Crenella. Stavelia. Dacrydium. Myrina. Idas. Septifer.