[Pediocactus simpsonii var. knowltonii (L.D. Benson) Halda]
Plants branched or unbranched. Stems globular to short cylindric, 0.7-5.5 × 1-3 cm; areoles circular, villous. Spines smooth, relatively hard, all radial, mostly 18-26 per areole, spreading, recurved, or somewhat pectinate, reddish tan, pink, or white, 1-1.5 mm, canescent. Flowers 1-3.5 × 1-2.5 cm; scales and outer tepals essentially entire, often undulate; outer tepals with brownish midstripes to 1.5 mm wide, 4-17 × 4-6 mm; inner tepals pink, oblanceolate, 8-25 × 3-8 mm. Fruits green, drying reddish tan, turbinate,4 × 3 mm. Seeds black, 1.5 × 1-1.2 mm, papillate but not rugose. Flowering spring. Gravel pavements in pinyon-juniper woodlands with mixed sagebrush; of conservation concern; 2000 m; N.Mex. Pediocactus knowltonii is the smallest and rarest member of the genus. The species is very closely related to P. simpsonii; chloroplast DNA sequence analysis provides support that it represents a recent developmental mutation within P. simpsonii (J. M. Porter et al. unpubl.).