Umbo

1. n. One of the lateral prominence just above the hinge of a bivalve shell.
2. n. The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.
3. n. 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.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.