Shell usually small (<10mm), white opaque or trans-lucent, suborbicular, inflated, equivalve, smooth or with fine comarginal sculpture, more or less equilateral, pallial sinus absent, periostracum fine and entire; umbos large, prosogyrous; lunule heart-shaped, usually sharply defined by incised margin, in some species escutcheon absent or ill-defined; ligament ex-ternal, opisthodetic, attached to nymphs, small re-silium present in some species; hinge with cardinals often scrolled, anterior cardinal of right valve short horizontal lamella at ventral mar-ginofhingeplateinterlockingwithsocketonlefthinge, posterior cardinal of left hinge tends to horizontal and in some species is more elongate than cardinal, set in from ventral margin, right and left antero-and postero-dorsal margins interlock with fine groove and ridge.
Mantle margin typically with 3 folds, pedal gape extensive, exhalant aperture and in some species inhalant aperture separated by narrow tissue junctions of innermuscular fold,small exhal antretractile siphon developed from inner muscular fold may be present, both apertures with small papillate tentacles on the middle sensory fold, no siphonal embayment; anterior and posterior adductor muscles relatively large and subequal; gills flat, homorhabdic, inner demibranchs extensive interlocking anterior and posterior to foot, outer demibranchs reduced in size and may be reflected; labial palps extremely small, foot moderately well-developed, with heel, small byssal gland present in some species; sexes separate.