It is suggested that nuculoid pelecypods show a primary evolutionary branching into two distinctive groups, and that two superfamily-level taxa, Nuculacea and Nuculanacea, represented by living Nuculidae and Nuculanidae, should be recognized. A specialized separated ligament pattern has probably been evolved independently within each group; thus, ligament pattern is an unsatisfactory criterion for basic phylogenetic subdivision, although it may be useful within each group. Most post-Paleozoic nuculoids can be clearly assigned; certain early Paleozoic nuculoids, however, do not resemble typical forms of either group and temporarily should be considered of uncertain higher affinities or assigned to an artificial superfamily.

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