umbonate

Having or resembling a knob or knoblike protuberance.

Having a boss or umbo, as a shield or disk of any sort.

In zoology: Formed into an umbo, a boss, or a knob; button-like; umbonal; umbonic.

Having an umbo, as a shell; bearing umbones of this or that kind; umbonated.

In botany, bearing an umbo or boss in the center, as the pileus of many species of Agaricus.