Cact. Succ. J. (Los Angeles) 41: 189. 1969.
Plants unbranched (rarely to 8 mature branches), white bristly spines obscuring stem. Roots dominated by a short or narrowly conic, carrotlike, succulent taproot, 1/3–1/2 of stem diam. Stems deep-seated, inconspicuous, obovoid, ovoid, or spheric to ± conic, 2.5–6 × 1–3.4 cm; tubercles 3–6 × 2.6–4 mm, moderately soft; areolar glands absent; parenchyma not mucilaginous; druses in pith and cortex abundant but nearly microscopic; pith 1/2 of lesser stem diam.; medullary vascular system absent. Spines (21–) 31–44 (–55) per areole, all snowy white, dark tips (tan to reddish-brown) on medium and large spines; radial spines (18–) 20–41 per areole, (4–) 6–9 (–10) × 0.03–0.2 mm; subcentral spines 0–12 per areole; outer central spines (1–) 3–9 (–17) per areole, appressed or (usually) slightly projecting; inner central spines (spines most likely to interpreted as central) 0 (–1) per areole, ascending, descending, or porrect, straight, longest spines (6–) 7–14 (–19) × 0.17–0.27 mm. Flowers nearly apical, 15–30 × 13–19 mm; outer tepals conspicuously fringed; inner tepals 14 per flower, margins white or cream (to light pink), midstripes pinkish to brown or brownish green, proximally white, 8–12 × 1.5–3 mm; outer filaments white or pinkish; anthers bright-yellow; stigma lobes usually 4, dark green to bright-yellow. Fruits bright red throughout, ellipsoid, cylindric, or clavate, 11–20 × 3–5 mm, not very succulent; floral remnant deciduous. Seeds black, ± spheric, ± 1.2 mm, pitted. 2n = 22.
Phenology: Flowering Feb–Mar(-May); fruiting mostly May.
Habitat: Limestone slopes, mostly in crevices of massive limestone outcrops
Elevation: 700-1700 m
Of conservation concern.
Coryphantha duncanii is inconspicuous, difficult to identify, and usually rare. Other coryphanthas in its range have thicker spines, except for C. chaffeyi and C. dasyacantha. Coryphantha duncanii resembles a stunted, crevice-limited growth form of C. dasyacantha but has caducous floral remnant. It is closely related to C. zilziana Boedeker of Mexico.