This crustose species has a bright yellow to slightly greenish thallus that is matt, smooth and contiguous to somewhat dispersed. Separate areoles are surrounded by a pronounced black prothallus and the areoles, in turn, form arcs around the apothecia. There usually is one apothecium per areole.
Medulla: K+ yellow
In Scotland, Rhizocarpon lecanorinum is found mainly in the Central Highlands where it grows on siliceous rocks and walls, especially when these are metal-rich. BLS distribution map
Rhizocarpon lecanorinum is distinguished from the similarly yellow Rhizocarpon geographicum by the more rounded rather than irregularly shaped areoles which form arcs around the apothecia.
scottishlichens.co.uk 