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.
Subjects: Plant Sciences and Forestry.