O. missouriensis (Missouri).—­A stout, prostrate kind, forming large, spreading masses under favourable conditions.  Joints broad, flattened, obovate, about 4 in. long by 2 in. wide, light green; spine-cushions less than 1 in. apart, and composed of numerous small, white spines, with from one to four longer ones; these latter fall away when the joints get old.  Leaves very short, with a little wool about their bases.  Flowers 3 in. in diameter, appearing from May onwards; petals yellow, dashed with rose, sometimes wholly rose-coloured or brick-red.  Stamens deep red; pistil yellow, with a conical stigma.  Fruit nearly round, spiny, about 2 in. long.  A native of Wisconsin, and westward to the San Francisco Mountains; introduced in 1814.  This species is as hardy as O. Rafinesquii, and thrives under similar treatment.  It has stood 22 degs. of frost without suffering, requiring only protection from rain in winter.  In North America it forms large, spreading masses on gravelly hillsides, and is much dreaded by travellers, and especially by horses; there it is usually covered with snow from Christmas to the following May.

O. monacantha (one-spined).—­A tall, robust plant, not unlike O. Dillenii in general habit.  It has flat, large joints, oblong or ovate in outline, rather thinly compressed, and bearing grey cushions over 1 in. apart, with a solitary spine, 11/2 in. long, springing from the centre of each cushion, and pointing downwards.  Flowers sulphur-yellow, 21/2 in. across, borne on the last-ripened joints in May, and abundant on well-grown plants.  Fruits ovate, 2 in. long, green, with tufts of short, brown bristles; pulp edible.  The species is a native of Brazil, but is now common in many tropical and sub-tropical countries.  It is a free-growing kind, soon forming a large specimen if planted in a bed of old brick-rubble, or other light, well-drained soil, and kept in warm greenhouse temperature.

O. nigricans (blackish); Bot.  Mag. 1557.—­Stem stout, erect, becoming hard and woody when old.  Joints flat, oval in outline, 5 in. to 8 in. long.  Cushions 11/2 in. apart, composed of short reddish-brown bristles and two or three long stout spines, which are yellow when young, but almost black when ripe.  Flowers produced on the young, ripened joints, orange-red, about 3 in. across and developed in August and September.  Fruit pear-shaped, rich crimson when ripe.  Introduced from Brazil in 1795.  This well-marked species thrives in a warm greenhouse.  It branches freely, and has a healthy aspect at all times.  It is represented at Kew by very large specimens; one of them, which was recently cut down, had a stem 12 ft. high and an enormous head of dark, green joints.  Its head was planted as a cutting.