Calylophus serrulatus
 Calylophus serrulatus is a species of flowering plant in the Onagraceae known by the common name yellow sundrops. Other common names include halfshrub sundrop serrate-leaved evening primrose shrubby evening primrose plains yellow primrose and halfleaf sundrop. It is native to central North America including central Canada and the central United States.This plant is a subshrub which branches and forms a bushy clump up to 18 inches tall. The toothed leaves are up to 3.5 inches long.