Originally: of, relating to, or resembling bivalve molluscs of a type represented by the nut shell Nucula. In later use: specifically of, relating to, or designating protobranch bivalve molluscs of the order Nuculoida, which includes mainly deposit-feeding species attested in the fossil record from the Ordovician period onwards, and often regarded as among the most primitive extant bivalves.
A nuculoid mollusc.
Mid 19th century. From scientific Latin Nucula, genus name, use as genus name of classical Latin nucula small nut + -oid.