Shell slightly convex or flattened, often gaping ventrally, umbones median or submedian. Hinge edentulous or with one tuberculiform cardinal in each valve, sometimes with a second and smaller cardinal present; a short posterior lateral is often present. Internal ligament opisthodetic, in a small pit or groove. The expanded mantle more or less covers the exterior of the shell.

The shells of the Galeommatidae are usually thin, brittle and featureless. The hinge is edentulous, irregular or often with one stout cardinal in each valve, sometimes with a second and smaller cardinal present in the left valve. The shell is more or less internalized by the profuse development of the mid-mantle folds, which are wrapped over the shell and, when fully extended, may cover both valves, sometimes permanently. In most species the mid-mantle folds can be withdrawn completely between the valves when the animal is disturbed. Inner folds of the mantle are fused posteroventrally to separate a wide anterior inhalant-pedal gape from a much smaller posterodorsal exhalant aperture, which is often raised on a short siphon. Anteriorly, the free and often frilled edges of the mantle folds normally extend beyond the shell edges as an extensive cowl or hood. It forms a poorly defined inhalant siphon for the anterior-to-posterior water flow through the mantle cavity. When the mantle is fully extended over the shell, the shell is only exposed at a minute oval opening above the umbones or at a slit in the mid-lateral region.

The exposed parts of the mantle may be completely naked, but are normally ornamented with sensory tentacles or papillae, whose tips may be pointed or blunt and are sometimes pigmented (whitish, yellow or red). The distinction between papillae and tentacles is based on size and length rather than on histological criteria. Both can be extended by swelling of interior haemocoelic spaces and shortened or withdrawn by contraction of longitudinal muscle fibres (Morton, 1973; O’Foighil & Gibson, 1984; Mikkelsen & Bieler, 1989). Two fairly large unpaired tentacles arise dorsal to the anterior hood and to the exhalant aperture in many species and in some are the only tentacles present. They are called anterior and posterior siphonal tentacles. In some species (e.g. Scintilla longitentaculata sp. nov.) they can be everted suddenly from the normal retracted position and waved vigorously, probably to divert or dissuade potential predators, and are then known as dymantic tentacles (Morton, 1975).

In species of Galeomma, the papillae may have a terminal swelling which autotomizes and, it is believed, releases a noxious substance which may deter would-be predators (Morton, 1973). Preserved specimens are often more or less retracted into the shell, with the tentacles protruding only slightly beyond the edges of the gaping shell. Only Aclistothyra has the mantle edges fused along the dorsal mid-line and is unable to retract.

The muscular foot is adapted for active crawling, as described by several authors (e.g. Arakawa, 1960, 1961). A byssus gland is present and a few fine byssal threads attach the bivalves to the underside of stones, crevices, coral galleries and other protected substrates. A number of species are commensal with burrowing crustaceans or echinoderms. Some species are largely solitary, while others are gregarious, gathering in small groups that may sometimes consist of more than one species.

The eggs are incubated in the suprabranchial chamber, which includes both inner and outer demibranchs. Most of the species which have been studied are hermaphroditic.

The systematics of this family is in need of revision, with emphasis on both shell and soft part characters. Our studies indicate that several species have been named many times, probably as a result of the old habit of studying only cleaned shells. We have followed a conservative line and hope that additional information will emerge from molecular studies of the specimens that we have collected at Phuket.