Fam. 6. Mesodesmatidae.—Mantle edges largely united, with three orifices, foot byssiferous or not; shell regular or irregular, usually one cardinal and strong lateral teeth. Tertiary——. Genera: Mesodesma, Ervilia.
Fam. 7. Mactridae.—External branchial fold directed ventrally, siphons fused, foot tongue-shaped; shell equivalve, triangular-oval, hinge with ligament in an internal fossette, another portion external, a bifurcated cardinal tooth in the left valve, fitting into a branching tooth in the right valve, laterals present. Jurassic——. Genera: Nactra, Harvella, Raëta, Eastonia, Heterocardia, Vanganella.
Sub-order III. Veneracea.—Branchiae slightly folded, foot compressed, siphons generally short, pallial line variable, two adductor muscles.
Fam. 1. Veneridae.—Siphons free or partly united, foot seldom byssiferous; shell solid, equivalve, hinge usually with three cardinal teeth, laterals variable. Jurassic——. Principal genera: Cytherea, Circe; Grateloupia (Tertiary), Meroe, Dosinia (= Artemis), Cyprimeria, Cyclina, Venus, Clementia, Lucinopsis; Thetis (Cretaceous), Tapes, Venerupis.
Fam. 2. Petricolidae.—Animal perforating rocks; shell oval, slightly gaping behind, two or three cardinals, no laterals, pallial sinus well marked. Recent——. Genera: Petricola, Naranio.
Fig. 307.—Cytherea dione Lam., Peru.
Fam. 3. Glaucomyidae.—Siphons long, united, foot small; shell produced, thin, hinge with three cardinals, no laterals, pallial sinus well marked. Recent——. Genus, Glaucomya (incl. Tanysiphon).
Sub-order IV. Cardiacea.—Branchiae much folded back, mantle edges with three apertures, foot cylindroidal, more or less produced, siphons present or absent, one or two adductor muscles, pallial line variable.
Fam. 1. Cardiidae.—Siphons rather long, foot long, no byssus; shell equivalve, more or less radiately ribbed, hinge with one or two cardinals in each valve, laterals variable, ligament external, two adductors. Brackish water or marine. Devonian——. Genera: Byssocardium and Lithocardium (Tertiary), Conocardium (Palaeozoic), Cardium (with many sections, including Hemicardium), Limnocardium (subg. Didacna, Monodacna, Adacna).