UMBO
Um"bo, n.; pl. L. Umbones, E. Umbos. Etym: [L.]
1. The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.
2. A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane; as, the umbo in the integument of the larvæ of echinoderms or in the tympanic membrane of the ear.
3. (Zoöl.)
Defn: One of the lateral prominence just above the hinge of a bivalve shell.
UMBONATE; UMBONATED
Um"bo*nate, Um"bo*na`ted, a. Etym: [NL. umbonatus. See Umbo.]
Defn: Having a conical or rounded projection or protuberance, like a boss.
UMBRA
Um"bra, n.; pl. Umbræ. Etym: [L., a shadow.]
1. (Astron.) (a) The conical shadow projected from a planet or satellite, on the side opposite to the sun, within which a spectator could see no portion of the sun's disk; — used in contradistinction from penumbra. See Penumbra. (b) The central dark portion, or nucleus, of a sun spot. (c) The fainter part of a sun spot; — now more commonly called penumbra.
2. (Zoöl.)