Take lead from large boss

• Convex shape
• Boss on a shield
• A slight rounded elevation where the malleus attaches to the eardrum
• Elevation where the malleus attaches to the eardrum
• Spiked shield boss
• Beak of a bivalve shell

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Umbo \Um"bo\, n.; pl. L. Umbones , E. Umbos . [L.] The boss of a shield, at or near the middle, and usually projecting, sometimes in a sharp spike. A boss, or rounded elevation, or a corresponding depression, in a palate, disk, or membrane; as, the umbo ...

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"boss of a shield," 1921, from Latin umbo "shield-boss, knob, projection."

Wikipedia Word definitions in Wikipedia
thumb|right|Cantharellula umbonata has an umbo. thumb|right|The cap of Psilocybe makarorae is acutely papillate. An umbo is a raised area in the center of a mushroom cap . Caps that possess this feature are called umbonate . Umbos that are sharply pointed ...

Wiktionary Word definitions in Wiktionary
n. 1 (context biology English) 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. 2 (context zoology English) One ...