Lecanora argentata (Ach.) Malme

    Thallus crustose, continuous, to 3-10 cm across, relatively thin, granulose to verrucose, more or less cracked, whitish, pale grey, grey-silver, pale grey-yellow, prothallus black, usually present, often visible, as greyish spot where thallus is thiner (check the hypothallus presence between thallus cracks, the prothallus seen can be the one of another species below this one). Apothecia 0.5-1.5 mm diam., sessile, usually not numerous and scattered, flat then slightly convex, disc brownish, brown-reddish, usually not very homogenous, hymenium sometimes lacking, leaving an empty bowl, rather glossy, pruine absent, exciple smooth, concolourous to the thallus, containing massive crystals not soluble in potassium and visible using polarized light microscopy, epithecium without granules nor crystals. Ascospores simple, broadly ellipsoid, 10-15 x 6-8 µm. Photobiont: Trebouxia. Thallus: C-, K+ yellow, P+/- pale yellow or P+ yellow. Rather common, on smooth barks of deciduous trees (notably Fraxinus).

_ Easily mistaken with Lecanora chlarotera of which the general aspect is very similar, but with a less smooth thallus, devoid of black prothallus and with apothecia colours more homogenous. Observation of apothecia sections, using polarized light microscopy, are often required to distinguish the two species.