Species: Rhizocarpon geographicum
Photos: On coastal rock. Ballyrisode and Dunmanus, Cork, Ireland.
Variable, often mosiac-forming. Glossy thallus with bright yellowish-green to yellow angular areoles. Conspicuous black prothallus. Innate, irregular-shaped black apothecia to 1.5mm diam. Epithecium colour is variable; chestnut and K+ crimson to dark-green K+/-.
Occurs on hard siliceous rocks, also found on roofing slates. Most frequent in upland and coastal areas.
Medulla: K-, C+/- red, P+/- yellowish-orange, UV+ bright-orange
Immediately above: Rhizocarpon geographicum. Dunmanus Bay, Co.Cork. February 2009