Cact. Hort. Dyck. 1849, 236. 1850.
Shrubs, forming clumps, to 0.4 m, often from tuberlike rootstocks. Stem segments not easily detached, dark green, cuneate-obovate to commonly rhombic, 5–20 × 4–8 cm, fleshy but firm; areoles 4–6 (–8) per diagonal row across midstem segment, oval to subcircular, 2–4 mm diam.; wool tan. Spines (0–) 1–6 per areole, usually in distal 40% of stem segment, gray-white to redbrown, erect or reflexed, terete or proximal ones flattened, sometimes spirally twisted, longest to 60 mm, very slender, 0.2 mm diam. Glochids well developed in adaxial tuft, yellow or tan to brown-red, aging dull brown, to 6 mm. Flowers: inner tepals red throughout or with red basal portions, 25–35 mm; filaments pale-yellow; anthers yellow; style whitish; stigma lobes cream to yellow. Fruits green to yellowish to dull red, long stipitate, elongate-obovoid, 25–40 × 15–40 mm, fleshy, base not narrowed, glabrous, spineless or nearly so; umbilicus deep; areoles 14–26. Seeds tan to gray, subcircular, 4–5 mm, thickish, warped; girdle protruding to 0.5 mm. 2n = 44.
Phenology: Flowering late spring–early summer (May–Jul).
Habitat: Plains, hills, sandy or loamy soils
Elevation: 900-1900 m
Ariz., N.Mex., Tex., Mexico (Chihuahua)
Plants of Opuntia pottsii that are intermediate morphologically to O. macrorhiza are known in Arizona.
"thickish" is not a number.