Caloplaca flavocitrina  (Nyl.) H. Olivier

    Thallus crustose, more or less continuous, composed of small flat areeoles, slightly flexuous with a sorediate margin, soredia potentially covering the whole thallus beginning from the margin, typically lemon-yellow, green-yellow, soredia usually paler becoming farinose and grey-greenish when old. Apothecia adnate, at least at first, 0.3-0.8 mm diam., disc flat, usually slightly more orange than the thallus, exciple concolourous to the thallus, more or less persistent, smooth or a little sorediate. Ascospores polarilocular, 9-13 x 5-7 µm , septum to 2.5-4  µm thick. Thallus and apothecia K+ purple. Common on alkaline substrata: calcareous rocks, cement, mortar, concrete, etc. much less frequently on alkaline tree barks. This taxon belongs to the Caloplaca citrina agg. group and some authors do not separate them. Here we consider this taxon according to the usage of British authors.  Differs, though sometimes rather difficult to observe, from Caloplaca citrina agg.  by its farinose soredia when old, paler than the cortex, its apothecia with smooth (or almost smooth) exciple and its growth on rocks as well as on barks. Caloplaca arcis has blastidia and the thallus margin is minutely lobate.