Description: This ice-white to chalk-white crustose species with smooth thallus and black, non-frosted discs arranged in almost concentric rings outwards from the centre occurs on slightly basic, slightly acid rocks, a habitat referred to by lichenologists as ‘intermediate’. It has a scattered distribution in Ireland, both in urban and rural areas.

Key Identification Features:

• The colour of the thallus and concentric arrangement of the black discs should be sufficient to alert one to the possibility of this species
• The similar Rhizocarpon umbilicatum, which also occurs in Ireland, has randomly distributed discs, a snowy-white thallus and is only found on quality limestones, never intermediate rocks.

Original text submitted by Simon Davey

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

iNaturalist: Rhizocarpon petraeum at iNaturalist World Species Observations database.