Lecania cyrtellina (images of British lichens)

Thallus a very thin, filmy, smooth, greenish-white to pale grey crust; apothecia to 0.3mm diameter, sessile, discs convex, translucent white to pink or brown, paler colours distinctly green when damp, ascospores 1-septate (a few spores, according to literature, may be 3-septate), ellipsoidal, 8-12×2-3.5(-4)µm. Widespread, local or overlooked, forming small, inconspicuous colonies on nutrient-rich bark and twigs.

Refs: Smith et al. (2009), 458; Purvis et al. (1992), 288; Frahm et al. (2010), 46-7 (photo).

Paler, smaller apothecia than L. cyrtella, and with distinctly smaller ascospores; L. naegelii can be very similar but has a thicker, more-defined thallus and much longer, 3-septate ascospores. All three species can occur together.

lastdragon.org

Images of British Lichens