Arthoniaceae Arthonia fusca (Arthonia lapidicola) Growth type crustose Chemical results negative Widespread on calcareous rocks and mortar in open habitats and also found on church windowsills below metal grilles. The thin, greyish, pale fawn or olive-brown thallus is partly immersed or irregularly scurfy. Dark brown or blackish apothecia are usually 0.2-0.4 mm in diameter, spores 1-septate, 11-15(-18) x 4-7 µm. Pycnidia can be few or numerous, the conidia bacilliform. Microscope photograph below.

Microscope photograph

Arthonia lapidicola
Wimborne Minster, Dorset. March 2017