Species: Rhizocarpon petraeum
Photos: On coastal rock. Ballyrisode beach, Cork, SW Ireland.
Chalky-white cracked and areolate thallus with a dark prothallus. Irregularly-shaped black apothecia (to 1mm) with paler margins, often arranged in concentric rings. Green-brown epithecium. Spores muriform, initially colourless but dark when old, 20-50 x 13-25 Ám.
Occurs on intermediate rocks (slightly-basic to slightly-acidic) and is found in both urban and rural areas.
Medulla: K+ yellow, P+ yellow-orange
Similar: Rhizocarpon umbilicatum. Randomly scattered apothecia with smaller sub-muriform spores. Restricted to pure limestones.
Rhizocarpon petraeum. Ballrisode beach, Co. Cork. February 2009