Plants erect, usually unbranched, sometimes deep-seated in substrate in winter but never flat-topped. Roots diffuse. Stems unsegmented, pale to dark green or bluish green, ovoid, spheric, depressed-spheric, depressed hemispheric, cylindric, or elongate cylindric, 1-40(-45) × 1.8-15(-20) cm, occasionally glaucous; tubercles usually coalescent into ribs (rarely remaining as separate tubercles); ribs 10-17(-20), crests deeply notched above each areole, thus ribs tuberculate; tubercles ± prominent on ribs; areoles 8-20 mm apart, elliptic to pyriform, with fertile portion as short adaxial prolongation confluent with spine cluster or connected to spine cluster by very broad groove, woolly; areolar glands few or absent; cortex and pith highly mucilaginous. Spines 2-17(-29); radial spines 2-11(-18) per areole, usually white or gray, sometimes straw colored, brown, pink, purplish pink, or black, straight or curved; central spines (0-)1-6(-11) per areole, gray, white, yellow, straw colored, red, reddish brown, brown, pink to purplish pink or black, usually of 2-3 distinct types, 1 or more hooked (rarely none hooked), acicular or subulate, or both (ribbonlike and papery in S. papyracanthus), longest spines 7-15 mm. Flowers diurnal, borne at adaxial edge of areoles at or near stem apex or in short extension of spine-bearing areoles, campanulate or funnelform, 1-6.7 × 1-6(-7) cm; outer tepals with greenish lavender, reddish brown, yellowish brown, or purple midstripes, white, cream, gold, rose, pink, or purple margins, oblanceolate, 10-45 × 3-10 mm, margins entire or fimbriate; inner tepals erect to ascending white, cream, yellow, or pink to purplish, often with dark midstripes, lanceolate to oblanceolate, 15-50 × 4-12 mm, margins entire or fimbriate; ovary scaly, spineless; stigma lobes 5-12, pink, green, yellow, or creamy white, 1.5-3.5 mm. Fruits dehiscent along 2-4 irregular, short vertical slits above base, or through basal abscission pore, green, often turning tan, pink or red, cylindric to subspheric, 4.2-30 × 3.5-21(-25) mm, thin walled, fleshy, becoming dry at maturity, naked or with few broad, thin scales; pulp greenish to white, scant; floral remnant persistent. Seeds brown or black, 1.5-3 × 1.9-4.5 mm, glossy or shiny; testa papillate (rarely furrowed). x = 11.
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