The periplomatid shell is small to medium-sized (to 90 mm), thin-walled to solid, and irregularly oval to elongated or rhomboidal (the ventral margin has strong interdigitating projections in Albimanus). The anterior end is rounded, the posterior end truncate or ROSTRATE with the tip directed slightly dorsally. It is usually INEQUIVALVE (right valve larger and more convex, overlapping the left valve at the umbo), moderately inflated, and either not gaping or gaping anteriorly and posteriorly. The shell is usually INEQUILATERAL (umbones posterior), with PROSO- or OPISTHOGYRATE UMBONES. A dorsoventral UMBONAL CRACK or fissure is present in each valve, equal in length to the chondrophore, and covered by periostracum. Shell microstructure is ARAGONITIC "PRISMATONACREOUS" and three-layered, with a simple PRISMATIC outer layer, a lenticular NACREOUS middle layer, and a sheet-nacreous inner layer. TUBULES are apparently absent. Exteriorly periplomatids are white, iridescent where worn, covered by a thin adherent PERIOSTRACUM and sand grains (in some taxa, e.g., Cochlodesma; apparently not incorporated into the periostracum but simply adhering to the rough surface). Sculpture is smooth or pustulate with weak commarginal striae, rarely with radial ribs. LUNULE and ESCUTCHEON are absent. Interiorly the shell is thinly nacreous to chalky. The PALLIAL LINE has a (usually deep) SINUS. The inner shell margins are smooth or rarely broadly denticulate. The HINGE PLATE is EDENTATE in adults. The LIGAMENT is internal (RESILIUM) and OPISTHODETIC, set on prominent CHONDROPHORES that are supported behind by shelly buttresses (or "clavicles"), usually extending posteriorly toward the muscle scar. A strong U-shaped free LITHODESMA bridges the valves anterior and dorsal to the chondrophores in most taxa; Of fadesma instead has a proteinaceous pad; Takashia has neither structure. A secondary external ligament of fused periostracum unites the valves along the dorsal hinge margin.
Periplomatids are marine SUSPENSION FEEDERS, moderately deeply INFAUNAL in soft sediment. Certain morphological adaptations (e.g., elongated palps, hooded mouth, and hypobranchial glands) are associated with life in turbid waters. They lie horizontally with the more convex right valve uppermost, and with the siphons in separate burrows (in Cochlodesma, only the incurrent siphon reaches the surface, with the excurrent discharging into the subsurface sand); the interiors of the siphonal canals are hardened by mucus produced by the siphons. Peristaltic movements reported in periplomatid siphons seem to be associated with tube formation and/or PSEUDOFECES removal. Some species are passive burrowers, i.e., they cannot rebury once dislodged from the sediment; however, Periploma margaritaceum repeatedly reburies itself under experimental laboratory conditions. Predators include boring gastropods.
The family Periplomatidae is known since the Cretaceous and is represented by ca. 6 living genera and ca. 35 species, distributed worldwide in temperate and tropical seas (a few are boreal), from intertidal to abyssal zones.

Shell subnacreous, conspicuously inequivalve, nearly closed, edentulous; resilium internal, between two anteriorly or vertically directed resilifers, often buttressed, the lithodesma rarely wanting; ligament and area absent; umbos fissured; pallial sinus broad and shallow. Animal with siphons separated to their bases, naked and wholly retractile; monoecious.
Distribution: Cosmopolitan. Remarks: Fossil, Tertiary.