M. fissurata (fissured); Fig. 61.—­In appearance, this rare species mimics some of the Gasterias, and is so different from all the kinds hitherto described, that very few people unacquainted with it would suspect that it belonged to the same genus as M. elongata or M. dolichocentra.  Indeed, some botanists have made a separate genus of this and several other plants of the same peculiar appearance, calling them Anhalonium.  M. fissurata is like a whip-top in shape, the root being thick and woody, and the tubercles arranged in a thick layer, spreading from the centre, rosette-like.  A living plant in the Kew collection is 2 in. high by 4 in. wide, the tubercles being triangular in shape, 1/2 in. thick, wrinkled, with an irregular furrow on the upper surface.  The flowers grow from the middle of the stem, and are 11/2 in. wide, and rose-coloured.  Native of Mexico, on hard gravel or limestone soils.  We know of no plant in English collections, except that at Kew, which was introduced from Mexico in 1886.  It flowers in September and October.

[Illustration:  Fig. 61.  Mamillaria fissurata.]

M. floribunda (free-flowering).—­A French writer on Cactuses, M. Labouret, calls this a species of Echinocactus, but it resembles so closely another species included by him in Mamillaria, viz., M. atrata, that we see no good reason for separating the two into different genera.  M. floribunda has an irregular conical stem, about 5 in. high by 4 in. wide at the base, round nut-like tubercles the size of filberts, crowned with star-tufts of spines 3/4 in. long, stiff, and brown, about ten spines being set with their bases in a small disc-like pad of dirty-white wool.  The flowers are very numerous, covering the whole of the stem-top, from which they stand erect, so as to form a dense bouquet of rose-coloured petals.  Each flower is 2 in. long.  Native of Chili; introduced about 1835.  Flowers in summer.  This handsome kind will thrive in a window, and, if well supplied with fresh air, sunshine, and sufficient water to keep the soil moist, it will flower almost every year.  It must have no water in winter.

M. gracilis (slender).—­A small Thimble Cactus, remarkable for its proliferous stems, a single stem 2 in. high producing all round its upper half numerous, offshoots, which fall to the ground and grow.  In this way a tuft of stems is soon developed round the first one.  If these offshoots are removed as they appear, the stem will grow longer and stouter than it does when they are left.  Tubercles small, green, crowded; spines in a stellate tuft, short, curved, pale yellow or white.  Flowers as in M. elongata, to which this species is closely allied.  In window cases, or on a shelf in a cool greenhouse, it will grow and multiply rapidly.  Like the bulk of the caespitose, or Thimble Cactuses, it does not make much show when in flower; and it is only its stems, with their white stars of spines and clusters of little offsets hanging about them, that are attractive.  Native of Mexico; introduced about 1850.  There is a variety known as pulchella, in which the spines are of a yellow hue.