The isognomonid shell is small to medium-sized (to 150 mm), thin-walled, and irregularly quadrangular to narrowly dorsoventrally elongated, with weakly defined posterior (and rarely anterior) AURICLES. It is EQUIVALVE to strongly INEQUIVALVE (right valve less convex), laterally compressed to extremely flattened, and with BYSSAL NOTCH in the less-convex right valve but otherwise not gaping. Most species byssally attach with the less-convex right valve against the substratum (PLEUROTHETIC on the right valve). The shell is INEQUILATERAL (umbones anterior), with OPISTHOGYRATE UMBONES separated by a narrow CARDINAL AREA. Shell microstructure is a mixture of ARAGONITE and CALCITE, and two-layered, with a calcitic PRISMATIC outer layer and an aragonitic NACREOUS inner layer. The outer layer is often lamellar and readily cracks and flakes, especially in dried shells. The inner nacreous layer does not extend to the ventral margin, leaving a broad interior prismatic margin; in Isognomon bicolor, this layer is bordered by a thickened VISCERAL RIM that delimits the functional ventral body cavity when the shell is fully closed. TUBULES are absent. Exteriorly isognomonids are covered by a thin, nonpersistent PERIOSTRACUM. Sculpture is smooth or with irregular commarginal lamellae and in some cases with fine to moderate radial sculpture. LUNULE and ESCUTCHEON are absent. Interiorly the shell is nacreous near the umbones, with a wide ventral prismatic border, and prismatic inside the AURICLES (when developed). The PALLIAL LINE is ENTIRE and dis-continuous. The inner shell margins are smooth. The HINGE PLATE is straight and narrow, EDENTATE in adults (in Crenatuia and some Isognomon) or with weak teeth reduced to a single subumbonal denticle fitting into a socket in the opposite valve and/or one set of elongated posterior ridges; teeth are stronger in juveniles, often obsolete in adults. The LIGAMENT is MULTIVINCULAR and OPISTHODETIC, comprised of numerous, short, parallel internal portions (RESILIA) set in RESILIFERS, which decrease in size posteriorly and increase in number ontogenetically.
The animal is MONOMYARIAN (anterior ADDUCTOR MUSCLE absent); the posterior adductor muscle is large, central, and concave in cross section. The posterior pedal retractor muscles are usually large (absent in Crenatula) and insert centrally near or within the cavity of the posterior adductor; the anterior pedal retractors are small, distally divided, and inserted anterodorsally. Pedal protractor and elevator muscles are absent. The MANTLE margins are not fused ventrally and can be darkly pigmented; SIPHONS are absent. The in-ner mantle folds form a PALLIAL VEIL. Simple PALLIAL ("siphonal") EYES (cap-shaped or cellular, without lens) are present in hognomon (absent in Crenatuia) on the outer folds tinder the periostracum. HYPOBRANCHIAL GLANDS have not been reported. The FOOT is anterior, small, and digitiform. It has a BYSSAL GROOVE; the adult is strongly byssate except in sponge-dwelling forms (Crenatuia).
The LABIAL PALPS are small and trigonal. The CTENIDIA are large and partially encircle the centralized muscle bundles; like the mantle, they are be darkly pigmented in some species. They are FILIBRANCH or PSEUDOLAMELLIBRANCH (ELEUTHERORHABDIC), HOMORHABDIC or HETERORHABDIC, and not inserted into (or fused with) the distal oral groove of the palps (CATEGORY III association). CEPHALIC EYES are present in most species (absent in Crenatula). Distally the tips of the ctenidia are attached to the inner mantle fold, separating INFRA- and SUPRABRANCHIAL CHAMBERS; water flow is anteroposterior. The STOMACH is TYPE III. The MIDGUT is not coiled. The HINDGUT passes through the ventricle of the heart, and leads to a rectum with an ANAL FUNNEL. Isognomonids are GONOCHORISTIC and produce planktonic VELIGER larvae. The nervous system is not concentrated. STATOCYSTS are absent in adults. ABDOMINAL SENSE ORGANS are present and asymmetrical.
Isognomonids are marine or estuarine SUSPENSION FEEDERS. Most species are EPIBYSSATE on hard substrata (rock crevices, mangrove roots); Crenatula lives obligatorily embedded within sponges, growing in pace with the sponge to maintain water contact. Isognomon often forms extensive gregarious colonies on seawalls and other surfaces. Exterior surfaces of the valves can be encrusted by epibionts. The extreme flatness of the valves has been interpreted as an adaptation that minimizes resistance to strong water flow. Predators include boring muricid gastropods. The diet of Crenatula modiolaris (Lamarck, 1819) from Hong Kong has been shown to consist of diatoms and dinoflagellates; octopuses prey extensively on this species.
The family Isognomonidae is known since the Permian, is represented by 2 living genera and ca. 20 species, and is distributed worldwide in shallow subtropical and tropical seas; hognomon typically inhabits shallow depths, whereas Crenatula is deep to abyssal. Isognomon alatus is consumed in Jamaica as a substitute for the dwindling populations of harvestable oysters (Ostreidae). Isognomonids are often called "tooth pearl shells" because of their prominent multivincular ligaments and nacreous interior surfaces.