Fam. 2. Modiolopsidae.—Extinct; Silurian to Cretaceous; adductor muscles sub-equal. Modiolopsis.—Modiomorpha. Myoconcha.
Fam. 3. Pernidae.—Shell very inequilateral; ligament subdivided; mantle open throughout; anterior adductor absent. Perna. Crenatula; inhabits sponges. Bakewellia. Gervilleia; Trias to Eocene. Odontoperna; Trias. Inoceramus; Jurassic to Cretaceous.
Sub-order IV.—Pectinacea.
Monomyarian, with open mantle. Gills folded and the filaments at summits and bases of the folds are different from the others. Gonads contained in the visceral mass and generally open into renal cavities. Foot usually rudimentary.
Fam. 1. Vulsellidae.—Shell high; hinge toothless; foot without byssus. Vulsella.
Fam. 2. Aviculidae.—Shell very inequilateral; cardinal border straight with two auriculae, the posterior the longer. Foot with a very stout byssus. Gills fused to the mantle. Avicula; British. Meleagrina. Pearls are obtained from a species of this genus in the Persian Gulf, Indian Ocean, &c. Malleus. Several extinct genera.
Fam. 3. Prasinidae.—Shell inequilateral, with anterior umbones and prominent anterior auricula; cardinal border arched. Prasina.
Fam. 4. Pterineidae.—Extinct; Palaeozoic.
Fam. 5. Lunulicardiidae.—Extinct; Silurian and Devonian.
Fam. 6. Conocardiidae.—Extinct; Silurian to Carboniferous.