Fam. 7. Hippuritidae (= Rudistae).—Shell very inequivalve, externally as in Caprinidae, umbo central in left valve, no ligament proper, left valve with strong hinge-teeth and grooves, two adductor impressions on prominent myophores, shell structure of the two valves differing. Cretaceous only. Single genus, Hippurites (Fig. [309], B).
Fam. 8. Radiolitidae.—Shell inversely conical, biconical, or cylindrical, general aspect of Hippurites, umbo of left valve central or lateral, right valve with a thick outer layer, often foliaceous, umbonal cavity partitioned off by laminae. Cretaceous only. Genera: Radiolites, Biradiolites.
Sub-order V. Myacea.—Branchiae much folded back, mantle edges usually with three openings, foot compressed, siphons large, united or not, two adductor muscles, pallial line variable.
Fam. 1. Psammobiidae.—Siphons long, not united, foot large, not byssiferous; shell equivalve, long, oval, slightly gaping at the ends, ligament external, prominent, two cardinal teeth in each valve, no laterals, a deep pallial sinus. Jurassic——. Genera: Psammobia, Solenotellina, Sanguinolaria, Asaphis, Elizia, Quenstedtia (Jurassic).
Fam. 2. Myidae.—Pedal orifice small, siphons long, united in great part; shell inequivalve, gaping at one or both ends, periostracum more or less extensive, ligament internal, resting on a prominent shelf; hinge-teeth variable. Cretaceous——. Genera: Mya, Tugonia, Sphenia, Corbula, Lutraria (for which latter some propose a separate family).
Fam. 3. Solenidae.—Foot long, powerful, more or less cylindrical, no byssus, siphons usually short, united or not, branchiae narrow; shell equivalve, long and narrow, gaping at both ends, with periostracum, umbones flattened, ligament external, hinge-teeth variable.? Devonian——. Genera: Solecurtus, Pharella, Pharus, Cultellus, Siliqua, Ensis, Solen, Orthonota (?), Palaeosolen (?).
Fam. 4. Glycimeridae.—Pedal orifice very narrow, siphons long, united in great part, often covered with periostracum; shell more or less equivalve, gaping at both ends, hinge toothless or with two weak cardinals, ligament external; animal free or perforating. Cretaceous——. Genera: Glycimeris, Saxicava, Cyrtodaria.
Fam. 5. Gastrochaenidae.—Foot small, cylindrical, no byssus, branchiae narrow, siphons long; shell perforating or cemented to a shelly tube, gaping widely on the anterior and ventral sides, no hinge-teeth, a deep pallial sinus. Cretaceous——. Genera: Gastrochaena, Fistulana (tube with a median diaphragm, perforated by the siphons).
Sub-order VI. Pholadacea.—Mantle edges largely closed, siphons long, united, foot short, truncated, disc-shaped, ligament absent, two adductor muscles; animal perforating.
Fam. 1. Pholadidae.—Organs contained within the valves, ctenidia prolonged into the branchial siphon, shell more or less gaping, thin, dorsal margin in part reflected over the umbones, one or more dorsal accessory pieces, no hinge-teeth, an interior apophysis proceeding from the umbonal cavity. Jurassic——. Genera: Pholas, Talona, Pholadidea (posterior extremity of the valves prolonged by a corneous appendage, a passage to the long tube of Teredo), Jouannetia, Xylophaga, Martesia; Teredina (Eocene).