Description: The thallus of this species forms small, mirror-cracked (areolate) crusts – often among other thalli such as Rhizocarpon geographicum – on damp, siliceous rocks and boulders in north-east Ireland, but it should be searched for elsewhere, especially in the west. The areoles of the thallus vary from dark grey to brown. When the discs are sectioned, look for the two-spored asci and large dark spores. The epithecium is K+ purple-red.

Key Identification Features:

• Thallus small, grey to brown but forming larger colonies to 10cm diam.; discs, plentiful with two-spores/ascus; spores large: 45–68µm long, muriform
• Damp acid rock, often by lough shores.

Original text submitted by Vince J. Giavarini

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

iNaturalist: Rhizocarpon geminatum at iNaturalist World Species Observations database.