Shell variable, irregular and inequivalve when sessile, byssiferous when young; in most genera the byssus becomes modified to a calcified or horny plug passing through a foramen in the attached valve, and fastened to other objects, a condition which may be permanent or transient; ligament amphidetic, more or less internal, supplemented by an internal resilium, for which the crura serve as chondrophores, ali- or multi- vincular; hinge usually edentulous, rarely rugose, with amorphous interlocking rugosities; posterior adductor small, subcentral, in the sessile forms reinforced by the pedo-byssal muscles, which are modified for service as adductors.
Distribution: Warm and temperate seas. Remarks: Fossil, Devonian; Jurassic.

The anomiid shell is medium-sized to large (to 150 mm), thin-walled, circular to oval, flat, or folded, usually distorted according to the contour of the substratum. The lower right valve is usually flat, much thinner than the upper left valve, and has a small to large, oval, subumbonal hole (BYSSAL NOTCH or FORAMEN) through which the byssus passes; the dorsal margin of the hole is open in some taxa, fused in others (e.g., Pododesmus). It is INEQUIVALVE, with the upper left valve usually more convex, usually byssally cemented on (PLEUROTHETIC on) the right valve, compressed, and not gaping. The shell is INEQUILATERAL, with umbones displaced from the margin by overgrowth of the mantle lobes and shell, especially in the upper left valve. Shell micro structure is a mixture of ARAGONITE and CALCITE, and one- or two-layered, with a foliated calcitic outer layer (also with PRISMATIC calcite in the right valve) and a thin aragonitic COMPLEX CROSSED LAMELLAR inner layer that does not extend outside the pallial line and is absent in some, TUBULES are apparently absent. Exteriorly anomiids are translucent and variably colored, covered by a thin PERIOSTRACUM. Sculpture is weak and irregular, and can reflect that of the substratum, LUNULE and ESCUTCHEON are absent. Interiorly the shell is thinly NACREOUS or hyaline. The PALLIAL LINE is ENTIRE. The inner shell margins are smooth. The HINGE PLATE is short and EDENTATE in adults, but with various large tubercles and ridges. The LIGAMENT is internal (RESILIUM), elongated oval, and sits on a stalked or mushroom-shaped RESILIFER (CRURUM).
Anomiids are marine or estuarine SUSPENSION FEEDERS, usually permanently and tightly attached to hard substrata (e.g., shells or rocks) by the calcified byssus. Limpetlike Enigmonia aenigmatica (Holten, 1803) is unique in being mobile and semiterrestrial, moving about on mangroves and other hard substrata in damp atmospheres; it is tolerant of long periods of exposure and varies in color with its substratum.
The family Anomiidae is known since the Jurassic and is represented by 4 living genera and ca. 15 species, distributed worldwide mainly in temperate waters, from shallow to deep depths. The common name "jingle shells" is derived from the shell craft industry, wherein empty shells shaken together, as in a mobile, make a jingling sound.