Caloplaca microthallina (Wedd.) Zahlbr.
Thallus limited to small rounded microsquamules, 1-2 mm across, yellow-orange, yellow, lemon, clustured or scattered, isidia and soralia absent, often aligned along rock fissures. Lacks a prothallus. Apothecia scattered, to 1 mm diam., disc flat, becoming convex, yellow-orange more or less dark according to exposition, proper margin paler and minutely crenulate. Ascospores ellipsoid, polarilocular, 12-15 x 6-8 µm, septum 2-3 µm wide, about 1/4 of the length of the ascospore. Photobiont: trebouxioid. All parts K+ purple. Very common, on all types of coastal rocks, overgrowing or close to Verrucaria and notably Hydropunctaria (Verrucaria) maura of which it seems to be a parasite. This species is understated and systematically has to be searched in the black zone with a high probability of success owing to its high frequency. N.B. Specimen of Caloplaca marina with thallus underdeveloped can be confusing, but the discs of apothecia are more bright orange with a smooth margin.