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 be 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.

Flowering Feb-Mar(-May); fruiting mostly May. Limestone slopes, mostly in crevices of massive limestone outcrops; of conservation concern; 700-1700 m; N.Mex., Tex.

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.