Fam. 7. Ambonychiidae.—Extinct; Silurian and Devonian. The last two families are dimyarian, with small anterior adductor.
Fam. 8. Myalinidae.—Extinct; Silurian to Cretaceous; adductors sub-equal.
Fam. 9. Amussiidae.—Shell orbicular, smooth externally with radiating costae internally. Gills without interlamellar junctions. Amussium.
Fam. 10. Spondylidae.—Shell very inequivalve, fixed by the right valve which is the larger. No byssus. Spondylus; shell with spiny ribs, adherent by the spines. Plicatula.
Fam. 11. Pectinidae.—Shell with radiating ribs; dorsal border with two auriculae. Foot byssiferous. Mantle borders with well developed eyes. Pecten; shell orbicular, with equal auriculae; without a byssal sinus; British. Chlamys; anterior auricula the larger and with a byssal sinus; British. Pedum. Hinnites. Pseudamussium. Camptonectes. Hyalopecten; abyssal.
Sub-order V.—Dimyacea.
Dimyarian, with orbicular and almost equilateral shell; adherent; hinge without teeth and ligament internal. Gills with free non-reflected filaments.
Fam. Dimyidae.—Characters of the sub-order. Dimya; recent in abyssal depths and fossil since the Jurassic.
Order III. Eulamellibranchia
Edges of the mantle generally united by one or two sutures. Two adductors usually present. Branchial filaments united by vascular inter-filamentar junctions and vascular interlamellar junctions; the latter contain the afferent vessels. The gonads always have their own proper external apertures.