A crustose lichen with striking yellow, green-yellow or occasionally orange-yellow thallus and a pronounced black prothallus that also shows in between the angular areoles. It often forms mosaics. The apothecia are black, irregular in shape and innate.
Medulla: K-
In Scotland, Rhizocarpon geographicum is very common on hard siliceous rocks, from coastal to upland regions. BLS distribution map
Rhizocarpon geographicum can be confused with the bright yellow to slightly greenish yellow Rhizocarpon lecanorinum, but in the latter species the areoles tend to be more separated and form arcs around the apothecia.
Due to the strong pattern of lines formed by the black hypothallus that runs between the areoles and the black apothecia, Rhizocarpon geographicum is often referred to as the map lichen.
scottishlichens.co.uk 