GNU Collaborative International Dictionary of English

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1.       
   ‖Umbo (?), n.; pl. L. Umbones (#), E. Umbos (#). [L.]
   1. The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.
   
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   2. A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane; as, “the umbo in the integument of the larvae of echinoderms or in the tympanic membrane of the ear”.
   
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   3. (Zool.) One of the lateral prominences just above the hinge of a bivalve shell.
   
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