umbo

noun

1. A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane. For example the umbo in the integument of the larvæ of echinoderms or in the tympanic membrane of the ear.
2. One of the lateral prominences just above the hinge of a bivalve shell.
3. A bump or protrusion on the top of the cap.
4. The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike.