Megapterium oklahomense Norton, Rep. (Annual) Missouri Bot. Gard. 9: 153, plate 47, figs. 1–3. 1898; Oenothera macrocarpa var. oklahomensis (Norton) Reveal; O. missourensis Sims var. oklahomensis (Norton) Munz

Herbs glabrous. Stems several, unbranched, sometimes with shorter secondary branches, 5–30(–42) cm. Leaves green, usually flecked with reddish purple splotches or reddish tinged, (5.5–)7–11(–12.5) × (0.3–)0.8–2(–3) cm; blade usually elliptic to lanceolate-elliptic, sometimes linear, moderately thick, fleshy, margins usually undu­late, usually conspicuously denticulate to serrulate, rarely subentire, apex acute. Flowers: buds with unequal free tips (5–)6–10(–15) mm; floral tube (82–)90–140(–147) mm; sepals 35–60 mm; petals (38–)42–55(–65) mm; filaments 24–34 mm, anthers (15–)19–21(–25) mm; style (120–)140–190 mm. Capsules globose to narrowly ellipsoid, not twisted, wings (10–)15–20(–25) mm wide, body 35–55(–75) × 7–9 mm.2n = 14.

Flowering Apr–Jun(–Sep). Rocky, clay soil, open sites of fine-textured, red sandstone usually mixed with gypsum, on pure gypsum, on limestone; 200–500(–800) m; Kans., Okla., Tex.

Subspecies oklahomensis is known from Barber, Harper, Meade, and Montgomery counties in extreme southern Kansas south across central Oklahoma to Harmon County in southwestern Oklahoma, and Cooke and Knox counties in adjacent Texas.