Acarospora impressula Th. Fr. ss. auct. brit.
Thin thallus up to 3 cm diam., often in small patches, areolate; areoles rather small, 0.5-2 mm wide, contiguous, interlocking, sharply angular, becoming elongate, forming a pavement, dark-red to black-brown, flat to slightly convex. Apothecia deeply immersed, 2-4 per areole (Magnusson), 1-10 per areole (Clauzade and C. Roux), 0.2-0.3 mm diam. often punctiform; disc round or variously angular often irregular, concolorous with thallus, with a thin, darker swollen margin. Ascospores shortly ellipsoid, 3-4 x 2-2.5 µm according to british authors, (3-5 x 1,5-2 µm according to Clauzade and Roux). Photobiont: Chlorococcales. Chemical spot tests negative. Found on exposed siliceous rocks notably granite and possibly present near the sea (according to British authors) or on calcareous rocks more or less decalcified or weakly acidic or alkaline as a result of birds feces and mostly in mountains (according to French authors, Roux, C., personal communication). NB: As mentioned in the text, the interpretations of this species vary according to the authors. On the pictures presented here, chemical reactions were negative and the specimen seem to colonize granite small fractures on nutrient-rich rocks. N.B. As specified in the text, this is the interpretation of the British authors because for the French authors this name is to be reserved for a mountain species parasitic at the beginning of the crustoses species of the genus Aspicilia. N.B. It is possible that this taxon, which will have to change its name, belongs to the genus Myriospora.