Ochrolechia parella (L.) Massal.

    Thallus crustose, 0.5-2 mm thick, smooth or slightly verrucose, sometimes forming large patches more or less regular, to 20 cm across, whitish, cream, pale grey, pale beige, prothallus whitish with growth concentric ribs that, in case of mosaics, evolve in turgid scar-like strips. Very fertile, always covered with numerous apothecia, 2-5 mm diam., sometimes very crowded and, under compression, distorted, exciple thick and swollen, disc surface grey-beige tinged pink or beige-orange, markedly pruinose, whitish when young (and not granulose like in apothecia of Ochrolechia tartarea). Ascospores colourless, broadly ovoid, 45-65 x 25-40 µm. Thallus: C+ yellow or C-, K-, KC+ yellow, Pd-; apothecia exciple and disc pruine: C+ red, K- , KC+ red, Pd-. Photobiont: chlorococcoid. Not especially maritime, but very abundant on siliceous rocks near the seashore where it may form conspicuous large whitish patches covering important surfaces. Many close species, difficult to identify, are described but they are either rare or absent in Brittany. Keep in mind that some forms with apothecia discs whithout whitish pruine but conspicuous granulation, or strongly tinged pink-orange or brown-orange, may be mistakenly named Ochrolechia tartarea if chemical spot tests are not carefully done (in the past, this seems to have been the case in Brittany where this species was reported to be common, which is no more the case nowodays where it is much less frequent and more localized than Ochrolechia parella). This species is highly variable and several taxa may be taken into account under this name.

    Currently the following differentiation is admitted: Ochrolechia parella subsp. parella on acid and not calcareous rocks, Ochrolechia pallescens on rough bark of trees (Quercus, etc.). Ochrolechia szatalensis is a very close species growing on smooth bark of trees in shaded areas, thallus and apothecia:  C-, K-, KC-, P-.*