This is a rare Mediterranean Atlantic species of warm, moist, calcareous habitats. It has opposite leaves and is usually green, but may be brownish in colour. The only species it might be confused with are Southbya nigrella, which is much darker and has a number of microscopic differences, and Gongylanthus ericetorum, a rare acidophile of peaty or sandy soils and known only from Cornwall and the Scilly Isles.
It is quite a brittle and fragile plant which makes dissection difficult but the longer than wide leaves with no appendage growing from the leaf surface are usually enough to differentiate it from S.nigrella. The stems are less than twice as wide as tall, and contain a concentration of arbuscular mycorrhizal fungi in the central region.Read the Field Guide account