Three new species of Atla from calcareous rocks (Verrucariaceae, lichenized Ascomycota)

Three new species of Atla are described from calcareous rocks in northern Finland based on morphology and ITS sequences. All new species have dark brown spores but have smaller perithecia and spores than previously known brown-spored species of Atla. Atla tibelliorum sp. nov. resembles Sporodictyon arcticum, but has a dark brown exciple and larger spores. Atla oulankaensis sp. nov. and A. vitikainenii sp. nov. differ from A. alpina in the smaller perithecia and spores. Atla vitikainenii has an involucrellum often slightly diverging from the exciple. Atla oulankaensis is similar to A. vitikainenii, differing in slightly narrower spores and in having some involucrella incurving under the exciple or even enveloping the exciple. An unidentified ITS sequence from Canada obtained from GenBank most probably belongs to A. oulankaensis and another unidentified sequence from the USA to A. tibelliorum. The new results strengthen the conclusion that the genera of Atla and Sporodictyon cannot be separated by morphology alone. A key to all known species of Atla and Sporodictyon is provided.