Verrucariaceae Placopyrenium fuscellum (Previously Verrucaria fuscella) Growth type crustose. Chemical reactions negative Widespread on basic walls, mortar and calcareous rocks. Often initially parasitic on Verrucaria nigrescens. Sometimes host to Zwackhiomyces lithoiceae. Pale lead-grey or greenish thallus forming small islets among other lichens, prothallus absent. Mature areoles divided by thin lines, the thallus margin thick and can appear minutely subsquamulose. Very small perithecia, usually 1-10 immersed within each areole. Spores simple, 11-20 x 4-8 µm. Microscope photo below. Similar: Buellia aethalea. Verrucaria polysticta has a black, non-fimbriate prothallus and more prominent perithecia

Microscope photographs

Placopyrenium fuscellum
Dorset. September 2012 and January 2013