Caloplaca britannica R. Sant. In Laudon
Caloplaca arcis p.p.
Thallus entirely of small unequal squamules, convex, swollen, granular, to 1 mm diam., the marginal lobes radiating, yellow to orange squamules 0.1-0.3 mm, covered by small yellow often forming mounds spherical isidia to 0.1 mm diam., irregular in size; pale, indistinct prothallus. Apothecia widely scattered, to 0,3-0,6 mm diam., thalline margin persistent paler than discs; discs flat orange. Ascospores polarilocular, ellipsoid, 11-14 x 4-6 µm, septum 3-5 µm wide, about 1/3 of the length of the ascospore. Photobiont : trebouxioid. All parts K+ purple. Thermophilic species, on dry, nutrient-rich dusty crevices in the mesic-supralittoral zone on siliceous or calcareous rocky shores and also on sheltered birdperching rocks; often together with Verrucaria (Hydropunctaria) maura and Caloplaca marina ( see corresponding pictures), scarce or unoticed.
N.B. This species belongs to the Caloplaca citrina group. Several interpretations have been proposed (it would be Caloplaca citrina according to Netherland authors) but in practice it is rather difficult to distinguish from Caloplaca arcis (with blastidia and more angular ascospores, not especially maritime, growing on walls of old chapels) and also from Caloplaca citrina agg. (with sorediate squamules and growing on walls and calcareous rocks in humid environments) and above all from Caloplaca littorea (hypothallus entirely golden yellow, covered with digitate isidia and growing slightly above previous species on seashore substrata).