Plants branched; branches 1-several. Roots fleshy taproots, to 24 cm, 5-8 cm diam. Stems cylindric to ovoid-cylindric, commonly 5-15(-25) × 3.5-7(-10) cm, flaccid; tubercles 4 mm diam.; axils short woolly; cortex and pith mucilaginous; latex absent. Spines 21-64 per areole, dark or light colored, depending largely on substrate color, glabrous (to hoary); radial spines 30-46(-60) per areole, white, bristlelike, 6-10 × 0.09-0.15 mm, stiff; central spines 1-3(-4) per areole, porrect or strongly projecting, usually hooked, (6-)13-18(-25) × (0.2-)0.3(-0.4) mm; subcentral spines several, often 12+ per areole, radiating in all directions, often resembling supplementary ring of radial spines, barely distinguishable from radial spines, stouter, longer and dark tipped or purplish. Flowers 2.5 × 2.5-3.5 cm; outermost tepal margins long fringed; inner tepals pink to rose-purple, margins sometimes paler or white, at least proximally, 24-26 × 4 mm; stigma lobes yellow-green to green. Fruits bright red, ellipsoid or cylindric to clavate, (8-)15-30 × 5-10 mm, juicy only in fruit walls; floral remnant quickly deciduous, leaving conspicuous abscission scar. Seeds black, conspicuously strophiolate, 1.4 -2.4 × 1.4 mm, pitted and rugose; testa hard; anticlinal cell walls straight (not undulate; interstices narrower than pit diameters; pits bowl-shaped; strophiole tan, large, corky. 2n = 22. Flowering Apr, Jul; fruiting Feb-Apr, Sep-Oct. Mojave and Sonoran Deserts, alluvium and outcrops, valley floors, hills, mountainsides; 100-1500 m; Ariz., Calif., Nev., Utah; Mexico (Baja California, Sonora). Mammillaria tetrancistra extends farther into hyper-arid California deserts than any other species of Mammillaria. Without the unique seeds, its identification requires detailed comparison with both Mammillaria grahamii and M. viridiflora. Although M. viridiflora is eco-geographically segregated (more mesophytic), the other taxa grow intermingled at many sites in southwestern Arizona. Pushing the side of the stems with a stick or stone allows crude field identification for two commonly confused species: stems of M. tetrancistra are soft and flabby, whereas stems of M. grahamii are firm.

Plant: Stem generally 1, 7-25 cm, 3.5-7.5 cm diam, cylindric; axillary bristles 0; ribs inconspicuous; tubercles round in X -section Leaves: Spines: central spines 3-4, 18-25 mm, generally hooked, tips dark; radial spines 30-60 in 2-3 ranks, 10-25 mm, white or tips dark Flowers: bisexual, sessile, ± radial, 30-40 mm, 25-40 mm diam; perianth deep pink to lavender; ovary glabrous; stamens many; ovary appearing inferior, ± submerged in stem Fruit: 15-32 mm, cylindric; red, glabrous; Seed: black, pitted, aril corky Misc: Sandy hills, valleys, plains, creosote-bush scrub; 130-1400 m.; Apr