Gypsum cactus, Siler’s pincushion cactus

Echinocactus sileri Engelmann ex J. M. Coulter, Contr. U.S. Natl. Herb. 3: 376. 1896; Utahia sileri (Engelmann ex J. M. Coulter) Britton & Rose

Plants few branched or un-branched. Stems depressed-ovoid or occasionally elongate-cylindric, 5-15(-25) × 6-11.5 cm; areoles circular to pyriform, villous to lanate. Spines distinguishable as radial and central: radial spines 11-15 per areole, white, 11-21 mm; central spines 3-7 per areole, nearly porrect, brownish black aging to pale gray or white, straight or slightly curving at tips, 13-30 × 1 mm at base. Flowers 0.8-2.2 × 2-3 cm; scales long fringed; outer tepals brown with white margins, long fringed, 9-15 × 3-4.5 mm; inner tepals yellow with purple veins, 15-20 × 4.5-6 mm. Fruits greenish yellow, short cylindric, 12-15 × 6-9 mm. Seeds gray to nearly black, 3.5-5 × 3-3.5 mm, papillate and rugose.

Flowering spring. Rounded hills in gypsum clay and sandy soils of Moenkopi Formation, Great Basin desert scrub; of conservation concern; 900-1600 m; Ariz., Utah.

Pediocactus sileri stems tend to develop a basal thatch of spines that anchor it to the fine, gypsum-rich soil. This species is endemic to a narrow strip along the Arizona-Utah border.