Rhizocarpon disporum

Rhizocarpon disporum, the single-spored map lichen, is a species of saxicolous (rock-dwelling), crustose lichen in the family Rhizocarpaceae. It has a circumpolar distribution and grows on sun-exposed siliceous rocks. Despite its species epithet, the lichen invariably contains only a single ascospore per ascus. A morphologically similar species, Rhizocarpon geminatum, has two spores per ascus; it is more widely distributed than R. disporum and tends to grow in moister habitats....read more on Wikipedia.