Shell not nacreous within, oval, compressed, smooth or concentrically striated, gaping at both ends, subequilateral; ligament external, elongated, resting on nymphs; hinge subhorizontal, formed by an anterior and posterior series of very fine teeth; no resilium; pallial line deeply notched behind; a linear depression extends from the umbonal cavity to the anterior adductor scar.

Shell roundish or longish, sometimes posteriorly truncated or pointed, not nacreous. Hinge margin not distinctly angulate, with more or less numerous teeth. Ligament completely external. Statocysts open, with sand grains.

Shell aragonitie, homogeneous or crossed-Iamellar; interior porcelaneous, rarely nacreous. Subequilateral to inequilateral usually with elongate, compressed posterior end. Periostracum thin, polished, adherent. Sculpture absent. Anterior and posterior gapes present. Hinge plate weak, arched, with delicate taxodont teeth in two series, sometimes separated by plain area, without resilifer. Ligament external, opisthodetic to amphidetic, weak. Pallial sinus large. Foot papillated. medially grooved sole. Siphons present, inhalant and exhalant fused together. Hypobranchial glands absent. Gills protobranch. Labial palps large; palp proboscides narrow, long. Alimentary canal with type 1 stomach; three ducts leading to digestive diverticula; style sac not fused to mid-gut; intestine passing through ventricle of heart.
The family is probably related to the Nuculanidae, but the lack of a resilifer supports its separation. Known from the Ordovician, two living genera with approximately 15 species arc recognized; several extinct genera are also known. Present in all oceans, especially in deep water in soft, muddy substrata. Both genera are present in the northeastern Pacific.

MALLETIIDAE Malletiidae is a small family of bivalves made up of 2 genera and about 15 species, not including extinct species (Coan et al. 2000). These bivalves are found in all oceans, particularly in deep-ocean realms in muddy bottoms. The lack of a resilifer and a prominent external ligament support the separation of this family from its strong relationship to the Nuculanidae (Reid 1998; Coan et al. 2000). Shells are composed of aragonite, uniform or cross-lamellar. Interiorly the shell is porcelaneous and rarely nacreous. Sculpture is absent or weak, dentition is taxodont, and adductor muscle scars are subequal. The posterior end of the shell is typically elongate and compressed; the hinge area is somewhat weak for a Nuculoida but still distinct. A slight opening or gape is present at both ends. Ctenidial lamellae are larger than those of nuculanids and probably function as pumps (Reid 1998). In Texas Malletiidae is represented by 1 genus and 2 species whose size ranges from 7 to 10 mm (¼ to ½ in).