Plants usually unbranched (rarely to 12 branches), usually most protruding above ground level, stiff and strictly erect, densely covered with spines. Roots diffuse or short taproots, largest roots basally less than 1/5 of stem diam. Stems spheric to short cylindric, 3.5–15 × 3.5–9.5 cm; tubercles 9–14 × 4–7 mm, stiff; areolar glands absent; parenchyma not mucilaginous; druses in pith and cortex sporadic, some giant, to 1 mm, lenticular; pith 1/3–1/2 of lesser stem diam.; medullary vascular system conspicuous, diffuse, present even in immature plants. Spines 18–41 per areole, lightly pigmented, white to pale pinkish gray (varying geographically), brightest when fresh, weathering to gray, dark tips present on all or all but smallest spines; radial spines (12–)15–33 per areole, 7.5–16.5(–19) × 0.1–0.4 mm; subcentral spines 1–7 often present adaxially; outer central spines 1–11 per areole; inner central spines (0–)1(–2) per areole, porrect or ascending (other central spines usually not or weakly appressed), in bird’s-foot arrangement or radiating evenly in adults, straight, largest spines 13.5–20.5 × 0.3–0.6(–0.8) mm. Flowers nearly apical, 18–44 × 8–37 mm; outer tepals densely fringed; inner tepals 13–34 per flower, ascending or recurved only at tips, pale dull creamy yellow or pale greenish tan to pale yellow-orange or pale apricot, usually with well-defined darker midstripes of maroon, magenta, pink, brownish orange, or yellow-green, proximally reflective, mirroring anthers, 11–25 × 1.8–2.8 mm; outer filaments white, pale pink, or white with pinkish distal 1/2, not contrasting with inner tepals; anthers bright orange-yellow; stigma lobes 4–11, erect or ascending, white to greenish white, nonstigmatic parts slightly pink, 2.5–4.5 mm. Fruits pale green, exposed parts sometimes dull rose-pink, ellipsoid to ovoid, 12–31 × 5–11 mm, succulent; floral remnant on fruit strongly persistent. Seeds reddish brown, ± obovoid or comma-shaped, 1.4–1.8 mm, pitted. 2n = 22.
Isolectotype of Mammillaria vivipara (Nuttall) Haworth subvar. borealis Engelmann [family CACTACEAE]
Isolectotype of Mammillaria vivipara (Nuttall) Haworth subvar. borealis Engelmann [family CACTACEAE]
