Pertusaria lactescens (images of British lichens)

Thallus whitish to pale or yellowish grey, smooth to cracked-areolate, unreactive with sodium hypochlorite, KOH giving yellow to red reaction (as shown in photographs), surface initially minutely papillate-isidiate, eroding to become sorediate; apothecia, when present, small, clustered in areoles and divided. Widespread and evidently locally common on sandstone rocks and walls, but not well understood until recently.

Refs: Smith et al. (2009), 683; Purvis et al. (1992), 456 (but outdated); Dobson (2005), 327 (photo); Dobson (2011), 327-8 (photo); van Herk & Aptroot (2004), 290-1 (photo, close-up of soredia).

lastdragon.org

Images of British Lichens