A crustose species with a grey, cracked to areolate, thallus that may be evanescent. The apothecia are rust-red to red-brown and have roughened discs that, on maturing, become convex. When young, the margins are glossy and are only slightly paler, but they may eventually become excluded.
Thallus: K-. Apothecia: K+ purple
Caloplaca crenularia is widespread along the coast, and in parts of central, western and southern Scotland where it is found on siliceous and slightly base-rich rocks, including gravestones, in sheltered, damp situations. BLS distribution map
Caloplaca crenularia is distinguished from Caloplaca ceracea on basis of the apothecia not becoming waxy dull green-orange when wet, and the glossy margins.
scottishlichens.co.uk 