M. bicolor (two-coloured).—­One of the commonest of the Cactuses grown in English gardens, and one of the most distinct, owing to its short, silvery hair-like spines, thickly crowded on the ends of the small tubercles, completely hiding the stem from view.  The latter is from 6 in. to 1 ft. high, 3 in. in diameter, cylindrical, often branching into several thick arms, when it has a quaint appearance.  If kept free from dust, which may be done by covering the plant with a bell glass, there is much beauty in the stem; indeed, it is owing to this, rather than as a flowering Cactus, that this species finds favour as a garden plant.  The flowers are less than 1 in. in length and width, stellate, their colour deep purple; they are developed in June.  Although a native of elevated regions in Mexico (4000-5000 ft.), this plant thrives best when grown in a warm house.  There are several handsome and very old specimens of it in the tropical collection of succulents at Kew.  It is one of the easiest to manage, and will thrive in a warm room-window if exposed to bright sunlight and kept dry in winter.  M. nivea and M. nobilis are both varieties of this species.

M. chlorantha (greenish-yellow).—­A newly-introduced species with erect, cylinder-shaped stems, 6 in. high, clothed with numerous tubercles, which are tipped with clusters of long, silvery, interlacing, hair-like spines, and a few stouter blackish ones.  The flowers are described as greenish-yellow, so that they are not likely to add much to the beauty of the plant, which is recommended because of the attractiveness of its stem and spines.  It is a native of Mexico and Texas, whence it was introduced some two years ago.  It requires cool-house treatment, and should be kept free from dust, which disfigures the white spines.

M. cirrhifera (twisted).—­Like M. bicolor, this species owes its frequent occurrence in gardens to the symmetry and neatly-chiselled form of its stems, and not to any attraction possessed by its flowers.  It will thrive anywhere where the sun can shine upon it, if sheltered from severe cold and wet.  In a cottage window it may be grown, and kept for many years, without losing health or, on the other hand, increasing much in size.  Its usual height is about 5 in., by 4 in. in diameter.  The tubercles are angular at the base, and bear tufts of yellowish spines on their pointed apices.  The flowers are small, and bright rose-coloured, but only rarely produced on cultivated plants.  Introduced from Mexico in 1835.

M. clava (club-shaped); Bot.  Mag. 4358.—­In the size of its stem, and the large, brightly-coloured flowers it bears, this species may be compared with some of the Echinocactuses.  The stem is from 1 ft. to 11/2 ft. high, 4 in. wide at the base, narrowing slightly upwards; the tubercles are 1 in. long, and nearly as much through at the base, their shape that of little pyramids, and their tips bear each from eight to eleven stout, straight spines, pale brown, with