Arthonia varians (images of British lichens)

Lacks a thallus or its own photobiont (and arguably a "lichen" only as a member of a generally lichenised genus), occurs in the hymenium of Lecanora rupicola, taking over and blackening the apothecia, retarding their development. Also the cause of the blackening commonly seen in apothecia of Lecanora sulphurea, according to Whelan (2011). Ascospores (1-)2-3-septate (compare A. apotheciorum). Widespread and evidently common where the host occurs, but evidently under-recorded.

Refs: Smith et al. (2009), 170; Purvis et al. (1992), 87; Dobson (2005), 70 (photo); Dobson (2011), 76 (photo); Whelan (2011), 49 (photo, in Lecanora sulphurea).

lastdragon.org

Images of British Lichens