Species: Lecidea lithophila
Photos: On upland rock. Gougane Barra, Cork, SW Ireland.
Left identified by John Douglass
The cracked-areolate thallus is greyish, usually with patches that are stained rusty-red. Large, +/- flat apothecia are dark-brown to black, becoming browner when wet. Asci 8-spored. Spores simple, colourless, 11-15 x 5-6 Ám. Microscope photograph below.
Common in upland areas, often near streams on iron-rich siliceous rocks. Chemical reactions negative.
Similar: Tremolechia atrata and Rhizocarpon oederi