From ZooTerms (Dictionary of Invertebrate Zoology)
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umbilicus (noun; plural -lici; Latin umbilicus, navel): 1. A navel, or navel-like depression

2. (Mollusca: Gastropoda) A cavity formed around the shell axis between the faces of the adaxial wall of the whorls where these do not coalesce to form a solid columella. a. In conispiral shells opening at the base of the shell, excepting hyperstrophic type. b. Involute shells may have two umbilici, an upper or adapical and lower or abapical in asymmetrical types, and left and right in isostrophic types.