The crassatellid shell is small to medium-sized (to 115 mm), solid, and quadrangular to rounded trigonal, anteriorly rounded, with the posterior end ROSTRATE or truncate. It is EQUIVALVE or INEQUIVALVE (left valve slightly larger), compressed, and not gaping. The shell is EQUILATERAL to INEQUILATERAL (umbones slightly posterior), with PROSO-, OPISTHO-, or ORTHOGYRATE UMBONES. Shell microstructure is ARAGONITIC and two-layered, with a CROSSED LAMELLAR outer layer and a COMPLEX CROSSED LAMELLAR or HOMOGENOUS inner layer, TUBULES are present penetrating the inner layer (or occasionally both layers). Exteriorly crassatellids are covered by a thin to thick, brown, polished or fibrous PERIOSTRACUM. Sculpture is smooth or (in most species) with distinct commarginal (occasionally oblique) ribs or folds, and sometimes with an internal layer of very fine radial sculpture, LUNULE and ESCUTCHEON are distinct and often indented; in Crassinella, the lunule is narrow whereas the escutcheon is wide and lunulelike. Interiorly the shell is non-NACREous. The PALLIAL LINE is ENTIRE, although a slight change in curvature marks the position of the posterior excurrent aperture. The inner shell margins are smooth. The HINGE PLATE is strong, flat, wide, and HETERODONT, with to or three CARDINAL TEETH posterior to the resilifer, plus anterior and posterior laminar LATERAL TEETH; all teeth can be finely transversely striate or serrate. The LIGAMENT is ALIVINCULAR and AMPHIDETIC (although mostly OPISTHODETIC), set on NYMPHS posterodorsal to an internal portion (RESILIUM) that is in turn set on a large, trigonal RESILIFER. A secondary external ligament of fused periostracum extends anteriorly and posteriorly.
Crassatellids are marine SUSPENSION FEEDERS, shallowly burrowing or attaching to hard particles with byssal threads; some are EPIFAUNAL, lying on the substratum. Predators include octopuses, boring gastropods, and shell-crushing sharks and rays. Certain features (e.g., adult byssus, absent outer demibranchs, and brooding) of some small Crassinella suggest that their evolution involves PEDOMORPHOSIS.
The family Crassatellidae is known since the Devonian and is represented by 9 living genera and ca. 40 species, distributed worldwide mainly in shallow tropical and subtropical waters. The robust shells of large South Australian Eucrassatella have been traditionally used as hand tools by Aboriginal hunter-gatherers.