Caloplaca scopularis (Nyl.) Lettau
Thallus usually small, 1-3 cm (5 cm), rosette-shaped, lobate, not pruinose at the margin, tips tightly appressed with a trend to widen like open fan, gold-yellow, bright yellow-orange, bright orange when exposed to sun, yellow to yellow-greenish when shaded or on concrete. Apothecia usually numerous, scattered all over the thallus, 0.2-0.8 mm diam., sessile, constricted towards the base, disc plane, without pruine, orange, exciple thin, persistent, paler. Polarilocular spores, 1-septate with equatorial thickening greater than 1/3 the length of the spore and up to 1/2, 9-12 x 4-6 µm depending on the literature, 10-14 x 5-6 µm with a thickening of (4) 5-6 (7) µm according to our measurements. Photobiont: trebouxioid. All parts K+ purple. On nutrient-rich weakly acidic of sometimes weakly alkaline subtrata (most often Armorican sandstone, gneiss and sometimes crough concrete) in the aerohaline zone jsut above the mesic supralittoral zone where a quite similar species may be found: Caloplaca thallincola. But the latter has longer lobes and less sessile apothecia and is a parasite of Verrucaria maura. Ascospore microscopic observation is often required to identify the two species: Caloplaca scopularis has ascospores with septum width over 1/3 of ascospore length. See Caloplaca arnoldii subsp. obliterata that grows on vertical walls in the same zone, and characterized by convex and not overlapping lobes. N.B. This species is often subjected to parasites.
N.B. This species of the Caloplaca saxicola group with powdery lobes is diversely analysed according to authors, and we present here our interpretation mostly shared with British and irish authors.