What does "melon" mean?

Melon: (Zoöl.) A large, ornamental, marine, univalve shell of the genus Melo. Melon beetle (Zoöl.), a small leaf beetle (Diabrotiea vittata), which damages the leaves of melon vines. -- Melon cactus, Melon thistle. (a) (Bot.) A genus of cactaceous plants (Melocactus) having a fleshy and usually globose stem with the surface divided into spiny longitudinal ridges, and bearing at the top a prickly and woolly crown in which the small pink flowers are half concealed. M. communis, from the West Indies, is often cultivated, and sometimes called Turk's cap. (b) The related genus Mamillaria, in which the stem is tubercled rather than ribbed, and the flowers sometimes large. See Illust. under Cactus.