Pl. umbones, umbos. [L. umbo, umbōnis shield-boss, knob, projection, etc. Cf. F. ombon (in sense 1).]
1. The boss of a shield, usually in or near the center, and sometimes having a sharp point.
1721. Swift, Poems, George-nim-Dan-Dean’s Answ. to Sheridan, 33. Like the umbo of the Romans Which fiercest foes could break by no means.
1753. Chambers’ Cycl., Suppl., Umbo, in antiquity, the round protuberant part of a shield.
1851. D. Wilson, Preh. Ann., II. iv. 268. Many of the shields of the same period were made chiefly of wood and leather, with the central umbo of bronze.
1899. R. Munro, Prehist, Scot., vii. 240. Similar relics … were associated with the iron umbo of a shield.
2. A projection of a round or conical form; a knob.
1753. Chambers’ Cycl., Suppl., Umbunculus … was afterwards used to express the inequalities on the surfaces of flints and agates, which frequently are roundish and obtuse and represent a kind of umbones.
1832. Gell, Pompeiana, I. vi. 116. The hot-water bath … consists in a vase or tazza of white marble…. In the centre is a projection, or umbo, rising from the bottom.
3. a. Conch. The point at which a univalve shell, or each valve of a bivalve shell, is most protuberant.
sing. 1822. J. Parkinson, Outl. Oryctol., 228. Terebratula semiglobosa:… tumid, very smooth; umbo raised, margin entirely without plicæ.
1877. Sir C. W. Thomson, Voy. Challenger, II. i. 5. The carina is a handsome plate, very uniformly arched, with the umbo placed at the apex.
pl. 1824. Q. Jrnl. Sci., XVII. 16. The umbones, which are unusually small, have scarcely any convexity.
1849. Dana, Geol. (1850), App. i. 699. A byssiferous canal passing out of the umbos at the margin of the shell.
1870. Rolleston, Anim. Life, 54. The bivalve shell of the fresh-water mussel,… with its ligament and its umbones.
b. Ent. (See quot.)
1826. Kirby & Sp., Entomol., III. 368. Umbones (the Bosses), two moveable bosses surmounted by a spine, with which the Prothorax of the Coleopterous genus Macropus is armed.
c. Bot. The knob or prominence in the center of the pileus of a fungus.
1836. Berkeley, Fungi, in Smith’s Eng. Flora, V. II. 28. Agaricus rufus.… Pileus 3 inches broad, plano-convex, slightly or strongly umbonate with a depression round the umbo as the plant advances.
1871. M. C. Cooke, Handbk. Brit. Fungi, 186. Pileus 1–2 in. broad, purple brown,… umbonate, the umbo generally subumbilicate.
d. Zool. One of the perforated ambulacral plates of echinoderms.
1877. Encycl. Brit., VII. 630/1. The ambulacra … have near their outer edge small shield like spaces, umbones,… perforated by pairs of small orifices or pores for the protrusion of the feet.
4. Path. A central patch in an efflorescence or other affection of the skin.
1822–7. Good, Study Med. (1829), V. 559. Efflorescence in blushing patches;… often alternately fading and reviving; sometimes with a colourless umbo. Ibid., 625. Even the area partakes of the vesication and becomes an umbo.
5. Anat. (See first quot.)
1877. Burnett, Ear, 51. The lower end of the manubrium draws the membrana tympani inward very markedly, and forms that depressed spot in the centre called the umbo.
1902. Hughes & Keith, Man. Pract. Anat., III. 281. The membrane is concave externally, the umbo forming the deepest point of the concavity.
