Rhizocarpaceae Rhizocarpon petraeum Growth type crustose Chemical results: Medulla: K+ yellow, P+ yellow-orange Found in urban and rural areas on intermediate rocks (slightly-basic to slightly-acidic). Cracked, areolate thallus, chalky-white with dark prothallus. Irregularly-shaped black apothecia (to 1mm) with paler margins, often arranged in concentric rings. Green-brown epithecium. Spores muriform, colourless becoming dark when old, 20-50 x 13-25 µm. Similar: Rhizocarpon reductum

Rhizocarpon petraeum
On wall. Westbourne, Dorset. January 2013