The genus Melocactus includes around 40 cacti from Mexico, the West Indies, and northern South America. Many of these species are endangered, and plants in cultivation are almost always grown from seeds. They are slow-growing, often difficult to grow. They are globular plants, generally solitary, except if the growing tip has been damaged. When they are mature, the body stops growing and produces a crown on its apex named cephalium. This densely spined area is where the flowers and fruit will be produced. The cephalium can keep on growing for many years, and in some species can exceed the height of the body itself. The striking difference between the body and the cephalium, reminds of a cap, hence the name Turk's Cap Cacti given to many specimen in this genus. You are bidding on the plant size shown.