Melocactus matanzanus (Turk's head cactus, Turk's cap cactus, melon cactus), a cactus from Mexico, is a large ribbed, ball-shaped or cylindrical plant that develops a cap or "cephalium" on the top of the plant at maturity.
Melocactus matanzanus. See more pictures of cacti.
Melocactus communis, from Jamaica, which has a similar appearance, was one of the first cacti ever seen by Europeans; they thought that the plants were actually the fruit of a giant thistle. These plants originate from warm, moist, coastal areas; unlike most desert cacti, they can tolerate humidity.
Give them bright light, heavy soil with good drainage and warm nights. Drench and let dry.
Caring for your cactus: