Rhizocarpon distinctum

Rhizocarpon distinctum is a crustose lichen with a gray-brown to dark-brown areolate thallus, meaning its surface is divided into small cracked patches. It also has a black prothallus and small black apothecia, or spore-producing structures.

It grows on rock, especially siliceous rock, rather than forming leafy growth on soil. Recorded localities include Denmark, Sweden, Norway, Finland, and rocky sites in Ireland.

The species was described by the Swedish botanist and lichenologist Theodor Magnus Fries. Its taxonomic record places the description in Lichenographia Scandinavica in 1874.

Useful diagnostic features include a medulla that turns blue with iodine and an epithecium that gives a red-purple reaction with potassium hydroxide. Its spores are colorless and can be three-septate to submuriform, so a field photograph alone may not be enough.