Shell trapezoidal or rounded, with the posterior side longer; ligament usually multivincular; hinge typically taxodont, with the teeth in two similar series, meeting below the umbos, and approximately vertical to the margin of the valve. Animal having a stout, short, deeply grooved foot, very often with a byssus; two adductor muscles of about equal strength; the borders of the mantle bear compound pallial eyes; the labial palps are direct continuations of the lips.
Distribution: All seas, at all depths.

The arcid shell is small to medium-sized (to 125 mm), elongated quadrangular to oval, and thin-walled to solid. It is EQUIVALVE to slightly INEQUIVALVE (left valve larger), inflated, and in some species ventrally gaping (or at least indented where the byssus emerges). The shell is EQUILATERAL or INEQUILATERAL (umbones anterior), with PROSO, OPISTHO-, or ORTHOGYRATE UMBONES separated by a narrow to wide CARDINAL AREA. Shell microstructure is ARAGONITIC and three-layered, with a PRISMATIC outer layer, a CROSSED LAMELLAR middle layer, and a COMPLEX CROSSED LAMELLAR inner layer. TUBULES are present in some species through all shell layers except the outermost (periostracum). Exteriorly arcids are covered by a thick, fibrous, pilose (occasionally hirsute) PERIOSTRACUM. Sculpture is primarily radial, often with weaker commarginal ridges, or in some species cancellate. LUNULE and ESCUTCHEON are absent. Interiorly the shell is non-NACREOUS; in a few species, the inner margin of the posterior adductor muscle scar is reinforced by a raised MYOPHORIC RIDGE. The PALLIAL LINE is ENTIRE. The inner shell margins are smooth or denticulate. The HINGE PLATE is straight or slightly arched, TAXODONT, with numerous perpendicular to oblique teeth, rarely reduced to a few nearly horizontal ridges, and often diminished in size or absent below the umbones. The LIGAMENT occupies the entire cardinal area and is SIMPLE or more typically DUPLIVINCULAR (with superficial chevron-shaped grooves), and AMPHI-, PROSO-, or OPISTHODETIC.
The animal is ISOMYARIAN or HETEROMYARIAN (anterior ADDUCTOR MUSCLE smaller in, e.g., Bentharca and Bathyarca); the pedal retractor muscles are elongated and well developed, especially posteriorly where the larger posterior pedal retractors have repositioned the pericardial cavity close to the umbones. Pedal protractor muscles underlie the anterior adductor. The MANTLE margins are not fused ventrally except, in some, for a small posterior excurrent aperture; SIPHONS are absent. In some burrowing forms, INCURRENT and EXCURRENT APERTURES are formed by temporary appression of the mantle lobes. The MANTLE margins are muscular and nontentaculate; simple PALLIAL EYES (cup-shaped, without lens) on the outer folds have been reported for some species, covered by periostracum. Deepwater Bentharca and Bathyarca possess a pair of prominent posterior mantle flaps and mantle flap glands, presumed to protect the gills and help convey PSEUDOFECES from the MANTLE CAVITY. HYPOBRANCHIAL GLANDS have not been reported. The FOOT is elongated and deeply grooved ventrally. A BYSSUS is present in the adult, is often robust, emanates from a conical process within the BYSSAL GROOVE, and can be shed and resecreted for relocation purposes. The LABIAL PALPS are relatively small to medium-sized, with ridges restricted to the dorsal part. The CTENIDIA are FILIBRANCH (ELEUTHERORHABDIC), HOMORHABDIC, of about equal size, and not inserted into (or fused with) the distal oral groove of the palps (CATEGORY III association). The posterior third of each gill is attached to a muscular stalk originating on the ventral surface of the posterior adductor muscle. CEPHALIC EYES are present. Incurrent and excurrent water flow is mainly posterior, with a secondary anterior incurrent flow. The STOMACH is TYPE III. The MIDGUT is variable in length and degree of coiling. The HINDGUT passes either dorsal to or through the ventricle of the heart, and leads to a rectum with a free ANAL FUNNEL. In species of Area, the heart includes two lateral pericardia, each with its own auricle and ventricle, and divided by the elongated posterior pedal retractor muscle; the ventricles are medially connected. Intracellular hemoglobin is present in the blood (e.g., the Blood Cockle, Anadara granosa (Linnaeus, 1758)) in red blood cells unknown elsewhere in the Mollusca. Arcids are GONOCHORISTIC and usually produce planktonic VELIGER larvae; one species of Lissarca is known to brood its larvae. The gonad often extends branches into the mantle tissues. The nervous system is not concentrated. STATOCYSTS in adults are present or absent. ABDOMINAL SENSE ORGANS are present. Arcids are SUSPENSION FEEDERS and usually marine, rarely inhabiting estuarine or fresh waters (e.g., Indian Scaphula nagarjunai Ram & Radhakrishna, 1984)- They can be EPIBYSSATE on coral or rock, or (presumably secondarily) ENDOBYSSATE in sand or mud, or less commonly, rock-boring (e.g., Panamic Litharca lithodomus (G. B. Sowerby I, 1833)) by a combination of mechanical and chemical means.