They have the fleshy stems characteristic of the order, these being either globose, oblong or cylindrical, and either ribbed as in Melocactus, or broken up into distinct tubercles, and most of them armed with stiff sharp pines, set in little woolly cushions occupying the place of the buds.
They are dwarf, ribbed, globose or cylindrical plants; and the flowers, which are produced from the side instead of the apex of the stem, are large, and in some cases very beautiful, being remarkable for the length of the tube, which is more or less covered with bristly hairs.
The fruits are small, globose, about in.
The young shoots of P. domestica are glabrous, and the fruit oblong; in P. insititia the young shoots are pubescent, and the fruit more or less globose.
It has long, narrow, flat, obscurely keeled leaves, a deciduous spathe, and a globose umbel of whitish flowers, among which are small bulbils.