From ZooTerms (Dictionary of Invertebrate Zoology)
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umbo (noun; plural umbones, umbos; Latin umbo, knob or boss): 1. (Arthropoda: Crustacea) a. In Cirripedia, a portion of the plate from which successive growth increments extend. b. In bivalves, apical portion of either valve

See also: beak. 2. (Arthropoda: Insecta) In Coleoptera, an elevated knob on the humeral angle of the elytra. 3. (Brachiopoda) Apical portion of either valve containing the beak. 4. (Bryozoa: Gymnolaemata) In Cheilostomates, a blunt knob on the front wall of the ovicell. 5. (Mollusca: Bivalvia) That region of the valve surrounding the point of maximum curvature of the longitudinal dorsal profile; when not coinciding with the beak, extending to its base.