Myriolecis crenulata (Hook.) Sliwa, Zhao Xin & Lumbsch

    Thallus very thin and inconspicuous, forming a more or less granular veil, dull whitish, pale greyish, pale brownish. Apothecia usually isolated, seldom clustered and crowded, scatttered all over the thallus, sessile and slightly concave, 0.4-1 mm diam., disc concave to flat, smooth to pruinose, brown-blackish, pale brownish, greyish and almost always tinged blue or grey-blue owing to the pruine, exciple thick, prominent, raised, folded-like, typically interrupted in sections producing a conspicuous crenulate aspect, white, pruinose. Ascospores colourless, simple, oblong, 8-12 x 4-6 µm. Photobiont: Trebouxia. Chemical spot tests negative, On vertical walls of hard limestone. Not to be confused with Lecanora agardhiana which is rather similar but of which the apothecia are immersed in small holes or cupules, at least at first. Nor with Caloplaca albopruinosa of which apothecia margin is not crenulate and with Lecanora antiqua  which grows on old walls made of siliceous stones subjected to above cement leaching and of which apothecia discs are C+ orange.  N.B. Another form growing on hard limestone of the foreshore, Lecanora crenulata forma, is rather similar but apothecia are very numerous and smaller  with an exciple markedly crenulate.