Description: P. subradiatum is a distinctive species on the vertical Carboniferous limestones of northern and western Ireland. It is occasionally plentiful, forming small, dark brown to almost grey-black, matt, narrow, radiating arc-like crescents (the result of the earlier circular thallus dying away from the centre) on limestone cliffs or beneath low ledges wetted by seepage. In such situations it is often a companion species to the local Collema fragile. It prefers south or south-east-facing rock where heat retention is high.

Key Identification Features:

• The dark, narrow, arc-like thallus on dry limestone is distinctive
• Low (<2m high), south-east-facing, tiered cliffs with lip-like overhangs are the best sites to look for it.

Original text submitted by Vince J. Giavarini

Distribution Map from NBN: Placynthium subradiatum at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Placynthium subradiatum at iNaturalist World Species Observations database.