Shell ovate to trigonal, inequilateral, inflated, equivalve. Periostracum thin, adherent, polished. Sculpture absent or commarginal riblets and striae, rarely with radial striae. Lunule and escutcheon usually present. Hinge plate wide; taxodont teeth in two series separated by short, oblique to subvertical resilifer. Beaks opisthogyrate. Ligament amphidetic, internal. Ventral shell margin crenulate.
Known from the Cretaceous, with approximately 60 living species, usually small and shallow infauna of soft sediments found in all depths and cosmopolitan in distribution. The name is derived from the Latin nucula, a small nut; the gender is feminine. Two subgenera are present in the northeastern Pacific.