M. vetula (old).—­One of the small Thimble Cactuses, its stems seldom exceeding 3 in. in height by 11/2 in. in diameter.  Tubercles 1/4 in. long, conical, with a radial crown of fine, hair-like yellow spines, 1/4 in. long, and a solitary central spine, 1/2 in. in length, and coloured red.  Flowers terminal, just peeping above the tubercles; sepals and petals acute, yellow, 3/4 in. long; anthers yellow; stigma white.  An old garden plant, introduced from Mexico.  It flowers in May and June.  For its cultivation it may be treated as recommended for M. pusilla.

M. villifera (hair-bearing).—­Stem similar to the last, but usually proliferous at the base; tubercles angular, short, woolly in the axils, and bearing four rigid, short, reddish-brown spines on the apex.  Flowers pale rose, with a line of purple down the middle of each petal; they are developed near the top of the stem, in May.  Native country, Mexico.  This plant thrives if treated as recommended for M. pusilla.  There are several varieties known, distinguished by their paler or darker flowers, or by a difference in the length and arrangement of the spines.

M. viridis (green).—­Stem 4 in. high by 3 in. in diameter, proliferous at the base; tubercles short, four-angled, crowded in spiral rows, woolly at the base, bearing each five or six radiating hair-like spines on the apex, and one central erect one, none more than 1/4 in. long.  Flowers erect, on top of stem, with recurved, pale yellow petals, 1 in., long; they are produced in May and June.  Introduced from Mexico in 1850.  It may be grown in a sunny frame out of doors during summer, and on a dry, warm greenhouse shelf in winter.

M. vivipara (stem—­sprouting).—­A tufted, free-growing Thimble Cactus, producing its small stems in such profusion as to form a cluster as much as 3 ft. in diameter.  The small tubercles are hidden by the numerous radial spines, which are in clusters of about twenty; they are white, hair-like, stiff and 1/2 in. long; the central spines, numbering from four to six, are a little longer.  Flowers from apex of stem, 11/2 in. long and wide, and composed of about thirty fimbriated sepals and twenty-five to forty narrow petals; colour bright purple.  Fruit 1/2 in. long, pale green when ripe.  The flowers, which appear in May and June, usually expand after mid-day.  Native of Louisiana.  In the North-West plains and Rocky Mountains of North America this plant is abundant, often forming wide cushion-like tufts, which, when covered with numerous purple, star-like flowers, have a pretty effect.  In Utah and New York it is commonly cultivated as a hardy garden plant, bearing exposure to keen frosts and snow without suffering; but it would not thrive out of doors in winter with us, unless covered by a handlight during severe weather, and protected from heavy rains in winter.  It likes a strong, clayey soil.

M. v. radiosa (Fig. 73).—­This variety is distinguished by its larger flowers and shorter spines.