Nuculida is an order of small saltwater clams, marine bivalve mollusks, within the subclass Protobranchia. "Nuculida" is sometimes spelled "Nuculoida". This order contains a single superfamily, Nuculoidea, which encompasses two families, the extant Nuculidae and the extinct Praenuculidae. Clams in the family Nuculidae are commonly known as nut clams.