Calylophus Spach, Hist. Nat. Vég. 4: 349. 1835

Herbs perennial, short-lived or suffrutescent, or annual. Leaves: blade margins subentire or serrulate or spinulose-serrate. Flowers usually crowded near stem apex; buds quadrangular; floral tube 2–20 mm; sepals with keeled midrib; stamens in 2 unequal series, antisepalous set conspicuously longer, pollen 30–100% fertile. Capsules hard, straight or, sometimes, slightly recurved, tardily dehiscent 1/2 to throughout their length. Seeds obovoid or oblanceoloid, sometimes sharply angled. 2n = 14.

Species 3 (3 in the flora): w, c North America, n Mexico.

Subsection Calylophus consists of three species (four taxa), one of which (Oenothera capillifolia) occurs primarily in Texas, but extends to southeastern Colorado, southern Kansas, southern Louisiana, and northeastern Mexico, and is disjunct in northwestern New Mexico; O. serrulata occurs throughout the Great Plains to Alberta, Manitoba, and Saskatchewan east to Wisconsin, west to east-central Arizona, with one occurrence in Chihuahua, Mexico, at elevations 0–1800(–2100) m. Species of subsect. Calylophus have diurnal flowers.