Synonyms: Caloplaca irregularis H. Magn.; Caloplaca microthallina (Wedd.) Zahlbr.; Physcia microthallina (Wedd.) Arnold; Placodium microthallinum (Wedd.) H. Olivier
Distribution: C - Sar.
Description: Thallus crustose-microsquamulose, episubstratic, lemon-yellow to yellow-orange, consisting of scattered to imbricate, 0.3-0.5(-2) mm mm wide, rounded, flat to convex, sometimes isidia-like areoles/squamules, forming up to 2-3 cm wide patches, often on the thalli of maritime Hydropunctaria-species. Thallus paraplectenchymatous with thin-walled cells. Apothecia zeorine, yellow-orange, sessile and constricted at base, 0.4-1.2(-1.6) mm across, with a flat to convex disc, a paler, minutely crenulated proper margin, and a conspicuous, mostly uneven to crenulate thalline margin. Thalline exciple 25-125(-175) μm wide; proper exciple (25-)40-60(-75) μm wide, of radiating, thick- walled, tightly packed hyphae; epithecium yellow-orange, granular, K+ red; hymenium colourless, 75-90(-100) μm high; paraphyses simple or sparingly branched, somewhat moniliform in upper part, 1-2 μm thick at mid-level, the apical cells 3-6(-7) μm wide; hypothecium colourless, 30-80 μm high, rarely inspersed with oil droplets. Asci 8-spored, cylindrical-clavate, Teloschistes-type. Ascospores 1-septate, polarilocular, hyaline, ellipsoid, (10-)12-15(-17) x (4-)6-8 µm, the septum 3-6 µm thick ( c. 1/4 of spore length). Pycnidia immersed, orange. Conidia narrowly ellipsoid, 2.5-3.2 x 1-1.2 μm. Photobiont chlorococcoid. Spot tests: thallus and apothecia K+ red, C-, KC-, P-. Chemistry: thallus and apothecia with parietin (major), fallacinal, emodin, teloschistin and parietinic acid (minor), corresponding with chemosyndrome A of Søchting (1997).
Note: a mainly Atlantic species in Europe, also known from North America, confined to the salt-spray belt, in the mesic supralittoral zone where it often grows on Hydropunctaria-species; in Italy it is known from a single station in northwestern Sardegna (Punta Falcone).