Flavoplaca arcis (Poelt & Vezda) Arup, Fröden & Sochting  

= Caloplaca arcis  (Poelt & Vezda) Arup ss. auct. brit.

    Thallus crustose, often discontinuous and then composed of small areoles, rosette-like, squamulose and sometimes minutely lobate towards the margin, or continuous and cracked to areolate, sulphur-yellow, lemon-yellow, yellow according to exposition, prothallus indeterminate, thallus covered and powdery with large more or less spherical blastidia, 0.05-0.1 mm diam., concolourous. Apothecia scarce, innate at first then sessile, disc flat, yellow to orange, exciple thick, concolourous to the thallus, more or less powdery with blastidia, usually persitent. Asci 50-70 x 15-18 µm according to our measures. Ascospores polarilocular, colourless, broadly ellipsoid,  11-14 x 6-9 µm according to literature, 3-5 µm thickening toward the middle section, 11-15 (17) x 5-9 (10) µm according to our measures (central thickening identical). Paraphyses apices swollen, 4-6 µm across. Apothecia and thallus K+ purple. This taxon belongs to the Flavoplaca citrina group and is diversely considered according to authors. Here we share the interpretation of British authors. Rather common on calcareous or weakly acidic or siliceous rocks that have become alkaline and in particular on stone walls of chapels, sometimes on cement, often together with Lecanora albescens, Verrucaria macrostoma, Phaeophyscia orbicularis, etc..

N.B. Very similar to Caloplaca britannica and often confused or assimilated but this taxon ss. str. is theoretically strictly maritime and grows in the fissures and crevices of siliceous rocks together with Flavoplaca marina and Hydropunctaria (Verrucaria) maura s.l moreover its blastidia are smaller and isidioid but there seem to be intermediates between the two taxa.

N.B. Very difficult to distinguish from Flavoplaca citrina s.l., and notably  taxa belonging to that group like the sorediate  Flavoplaca flavocitrina of which the exciple is smooth, and from Flavoplaca limonia, also sorediate, on calcareous rocks.