Nematostylis was studied in detail by Puff et al. (1984), and includes one species of small shrubs that is relatively widely distributed in Madagascar. Nematostylis is distinctive in its rather succulent leaves, short cymose inflorescences, and flowers each with one enlarged petaloid red calyx lobe, the slender tubular-funnelform corolla bicolored with the lower part bright red to orange and the upper part bright yellow, and the style well exserted, often with the exposed portion as long as the corolla tube. The fruits are dry and indehiscent with one seed in each locule and an enlarged calyx lobe, which loses its bright color but remains attached to the fruits apparently to aid in dispersal.
Nematostylis is similar to the Madagascar genera Razafimandimbisonia and Carphalea, see the project web page for Razafimandimbisonia for a key to separate these genera. The MO specimens are on loan so the distributional information here is largely taken from Puff et al. (1984).