A crustose species with a thallus that usually is found in circular patches that may form mosaics. The thallus is grey to mouse-brown, cracked areolate, and is often surrounded by a thin, black prothallus. Initially innate, the apothecia often become sessile when mature. The discs are black, rough and with a persistent margin, but very mature apothecia may become convex and exclude their margin.
C-, K- or K+ yellow or K+ red
Rhizocarpon reductum is widespread across Scotland where it grows on siliceous rocks, and gravestones in particular. BLS distribution map
On gravestones, Rhizocarpon reductum often forms mosaics with Buellia aethalea.
scottishlichens.co.uk 