Lecanora helicopis (Wahlen.) Ach.
Lecania helicopis (Wahlemb.) Clauzade & Cl. Roux s.l.
Thallus crustose, highly variable according to expositions, 1-5 cm across and forming colonies, smooth to rimose, more or less cracked, grey, grey-brown, lead-grey, prothallus usually well-developed, dark grey-blue, blackish or paler whitish-cream (margins with both concentric colours, dull whitish and lead-grey are commonly found). Apothecia usually numerous, 0.2-0.5 mm diam., scattered all over the thallus, innate then sessile, disc flat then convex or slightly rounded, dark brown becoming brown-blackish, exciple thin, indeterminate, not crenulate, paler and concolourous to the thallus, blackening and finally excluded. Ascospores simple but guttulate and then appearing 1-septate, ellipsoid, 9-15 x 4-6 µm. Photobiont: Trebouxia. Chemical spot tests negative. Rather common on the siliceous rocks or boulders, at the upper level of Verrucaria (Hydropunctaria) maura agg. and potentially together with the latter (this association is rather typical) as well as with the Caloplaca species thriving in this habitat. Prefers less well-lit zones than Lecanora actophila, which much less common. Often associated (and sometimes confused in this case!) to Halecania ralfsii in areas where this- rare- species usually grows.
N.B. This species is close to Lecania atrynoides with 1- septate spores and these two species frequently coexist on low acid rocks, it is sometimes difficult to separate them even after a microscopic examination because Lecanora helicopis can have spores 1- septate.
N.B. Very variable species and which perhaps contains several taxa, see: Lecanora helicopis forme sciaphile for the forms coming in the little sunny zones and Lecanora helicopis forme héliophile for the forms coming in the very sunny zones.