Picture taken on the 20th January 2008 near Sant Jaume ermitage, Sant Cristofol, Montserrat, Barcelona, Catalonia, corticolous, on Morus sp.
Granulous and warted thallus, white or pale grey. Apothecia with a big thalline margin, crenulate, granulous, ephytecium without granules, but with big crystals.
Be careful not to confuse it with Gyalecta ulmi, very similar but with very pruinose apothecia.
Thallus C-, K+ yellow, P-
See good picture in: C. Perez et alter (2003) Guía dos liques de Galicia. Baía Edicións, pag. 218.