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Rhizocarpon (order Lecanorales) A genus of crustose lichens in which the apothecia are lecideine and normally black. Spores are pale to dark brown, 1-or 3-septate to muriform. All species are found on rocks. R. geographicum occurs on siliceous rocks and walls, mainly in hilly districts; the thallus is bright yellow, surrounded and intersected by fine black lines, and dotted with small black apothecia.