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Neogastropoda (class Gastropoda, subclass Prosobranchia) An order of prosobranch gastropods which may have one pectinibranch gill, or less commonly two. A proboscis develops and there is a large osphradium. The nervous system is concentrated. Sexes are separate. The shell has an obvious siphonal notch or canal (i.e. in contrast to the entire aperture in Mesogastropoda) and has an operculum. Most neogastropods are carnivorous. They first appeared in the Cretaceous.