Segments distinctly flattened, the margins rounded, often purplish, the margins deeply scalloped. Areoles without bristles. Flowers 1-4, about 1 cm wide, whitish.
R. micrantha (Kunth) DC. from Ecuador and N Peru has at least some stems flattened and with extremely shallow scalloping.
R. robusta Lem. from Brazil is similar but the segments are often 3-winged.
Source: (1997). Cactaceae. In: . Horticultural Flora of South-eastern Australia. Volume 2. Flowering plants. Dicotyledons. Part 1. The identification of garden and cultivated plants. University of New South Wales Press.