Description: To the untrained eye, this species can look like any other ordinary yellow Rhizocarpon crust, the commonest of which is R. geographicum. The crescent-shaped areoles of R. lecanorinum form a pattern against the rock that is quite unique, giving it a certain elegance and strange beauty. The areoles surrounding the disc resemble a thalline margin (car-tyre shaped) of the kind seen in Lecanora and allied lichens, hence the specific name. Also, using Pd on the medulla, you can separate it from R. geographicum in the field, which is Pd+ yellow; this species is Pd+ orange, the test for stictic acid. In Ireland, records range from County Cork to County Donegal. It is also present in County Wicklow, but should always be looked for on granitic rock.

Key Identification Features:

• Thallus yellow with black discs; areoles crescent-shaped; medulla Pd+ orange
• With R. geographicum; often on granite.

Original text submitted by Vince J. Giavarini

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

iNaturalist: Rhizocarpon lecanorinum at iNaturalist World Species Observations database.