Caloplaca arcis (Poelt & Vezda) Arup ss. auct. brit.
Caloplaca limonia ss. auct. méditerranéens ?
Caloplaca coronata ss. auct. germaniques ?
Caloplaca britannica ss. auct. neerlandais ?
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 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 Caloplaca citrina group and is diversely considered according to authors. Here we share the interpretation of British authors. Rather common on calcareous rocks and notably on stone walls of chapels, often together with Lecanora albescens, Verrucaria macrostoma, Phaeophyscia orbicularis, etc.. Very similar also to Caloplaca britannica that is strictly maritime and grows in crevices of siliceous rocks together with Caloplaca marina and with Hydropunctaria (Verrucaria) maura s.l.. Very difficult to distinguish from Caloplaca citrina s.l., and notably taxa belonging to that group like the sorediate Caloplaca flavocitrina of which the exciple is smooth, and from Caloplaca limonia, also sorediate, on calcareous rocks.
N.B. Considering the lack of consensus between authors here we present under the Caloplaca arcis name specimen that were collected on siliceous stones assembled with lime mortar of an old chapel near the seashore in Landévennec, together with Hydropunctaria (Verrucaria) maura s.l.. They also might have been named as Caloplaca britannica, owing to the latter globose isidia, but in practice hardly distinguished from blastidia. Collected specimen near the sea level presented all transition forms between the two species.