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 Verrucaria maura. 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.