The Glaucomyidae and the Solenidae are generally included in the suborder Solenacea, but the systematic position of the Glaucomyidae has always been doubtful. The habit and structure of Glauconome rugosa are here described and compared with those of the Solenidae as exhibited in the first place by the species Pharella acuminata. The systematic position of the genera comprising the family Glaucomyidae is discussed. The structure and mode of growth of the external, opisthodetic ligament is described and it is shown that the form and growth of the shell (which includes the ligament) in the Bivalvia can be represented satisfactorily only when the two valves together with the ligament are considered as a unit. Fusion of the mantle margins ventrally and the formation of the long, united siphons involves the inner marginal folds together with the inner surfaces of the middle folds. In association with the mode of life, the foot is poorly developed and the pedal gape restricted to the anterior end. The ctenidia are capable of dealing rapidly with water containing large amounts of sediment, while the ciliation of the frontal surfaces of the filaments and the currents on them are similar to those of Petricola pholadiformis. The style sac and mid-gut are combined. Also included in the family Glaucomyidae is the genus Tanysiphon and shells of Tanysiphon rivalis were examined. The ligament of this species is short and, unlike that of Glauconome rugosa, extends obliquely across the hinge-plate posterior to the poorly developed cardinal teeth. The relationships between Glauconome and Tanysiphon are discussed and it is concluded that the latter genus is closely allied to Lutraria and should be transferred to the Mactridae. Features, which unfortunately could not be verified, but which probably characterize the animal of Tanysiphon and would indicate it to be a member of the Mactridae, are listed. Of the three subfamilies previously considered as comprising the Solenidae only the Soleninae remain. The habits and general structure of the available genera now included in the Solenidae are described: in particular Pharella acuminata (in greatest detail); Siliqua patula (preserved specimens); Cultellus lacteus and C. subellipticus (shells only); Phaxus pellucidus; Solen marginatus; Ensis siliqua, E. arcuatus and E. ensis. In all members of the Solenidae the mantle/shell is elongated posterior to the demarcation line and in the more specialized genera this is accompanied by a marked reduction in depth. The external, opisthodetic ligament is composed of anteriorly and posteriorly secreted fusion layer and outer and inner layers of the primary ligament. A fourth pallial aperture is present in Siliqua, Cultellus (probably), Phaxus and Ensis and in these genera the inner marginal folds anterior to the fourth pallial aperture are joined by cuticular fusion. Posterior to the fourth pallial aperture there is complete tissue fusion of the mantle margins. This involves the inner folds only in Siliqua and the inner folds together with the inner surfaces of the middle folds in Phaxus and Ensis. In Pharella and Solen, tissue fusion is of this latter type along the entire ventral margin. In all genera the inner surface of the middle fold is involved in the formation of the siphons. The ctenidia range from flat and homorhabdic (Phaxus) to plicate and heterorhabdic and adjacent tracts of long and short cilia are present on the frontal surfaces of some (Phaxus) or all of the lamellae. The style sac and midgut are always separate. The most significant feature of the Solenidae is the posterior elongation of the mantle/shell, and the effect of this on pallial attachment and the position of the adductor muscles is discussed. In Phaxus, Solen and Ensis the primitive ventro-dorsal axis of the anterior adductor muscle is orientated antero-posteriorly. As a result, pallial attachment extends from the posterior end of the elongated adductor anteriorly to the anterior end of the functional ligament. The systematic position of the family Glaucomyidae (now represented by the single genus, Glauconome) is discussed. It is concluded that the Glaucomyidae are venerid bivalves specialized for life deep below the surface of the substrate. The mobility which characterizes the Veneridae has been lost, the animal living permanently embedded in the substrate and maintaining contact with the surface by way of the long siphons. The Glaucomyidae should be included with the Veneridae and Petricolidae in the Veneracea and not with the Solenidae in the Solenacea.