Order I. Protobranchiata
Branchial filaments not reflected, the two rows inclined at a right angle (more or less), ventral surface of foot more or less flattened, byssogenous apparatus little developed, a single anterior aorta, kidneys distinct, sexes separate, each genital gland opening into the corresponding kidney.
Fam. 1. Nuculidae.—Labial palps very large, rows of branchial filaments at right angles to one another, mantle edges open, siphons contracted, foot disc-shaped, elongated; shell equivalve, oval, or produced, interior generally nacreous, hinge with numerous saw-like teeth. Silurian ——. Principal genera: Nucula (heart dorsal to the rectum); Palaeoneilo (Devonian), (?) Sarepta, Leda, Yoldia, Malletia; Tyndaria (Upper Tertiary), Lyrodesma (Silurian), Actinodonta (Silurian), Babinka (Silurian).
Fam. 2. Solenomyidae.—Labial palps united, one row of branchial filaments pointing dorsally, the other ventrally; mantle edges in great part united postero-ventrally, a single siphonal orifice with two very long tentacles, foot proboscidiform, with a round denticulate disc at the end; shell equivalve, resembling a Solen, with a strong corneous periostracum; no hinge-teeth, ligament internal. Single genus, Solenomya. (?) Cretaceous ——].
Order II. Filibranchiata
Rows of branchial filaments parallel, pointing ventrally, reflected, and provided with interfilamentary ciliated junctions, foot usually with a well-developed byssogenous apparatus.
Sub-order I. Anomiacea.—Heart dorsal to the rectum, a single aorta, foot small, anterior adductor very small; shell ostreiform, no hinge-teeth, fixed by a calcified byssus traversing the right valve (Fig. [173], p. 262).
Fam. Anomiidae.—Jurassic ——. Genera: Anomia, Placunanomia; Carolia (Eocene), Placuna; Hypotrema (Jurassic), Placunopsis (Oolite).
Sub-order II. Arcacea.—Mantle edge open, both adductors well developed, heart with two aortae, branchiae free, without interlamellar junctions, no siphons; renal and generative apertures distinct.
Fam. 1. Arcadae.—Mantle edge with composite eyes; shell round or trapezoidal, solid, often with stout bushy periostracum; ligament often external, on a special area; hinge with numerous lamelliform teeth. Ordovician ——. Principal genera: Arca (incl. Barbatia, Scaphula, and Cucullaea), heart dorsal to rectum; Pectunculus, Glomus, Limopsis; Trinacria and Nuculina (Tertiary).