Thallus variable, yellow- to brown-green or pale green, of usually irregular to imbricate, concave to highly convex thick squamules, sometimes strongly pruinose at the thallus margins or throughout, the margins sometimes thickened, loosely to firmly attached; lobes 1.2-2 (-2.5) mm wide. Isidia and soralia absent. Photobiont chlorococcoid.
Anamorph: conidiomata pycnidia, with curved filiform conidia.
Teleomorph: ascomata apothecia, 3-4 mm diam., the thalline exciple almost entire, becoming excluded. Disc pale brown to reddish brown, initially concave to flat, becoming irregularly convex. Hymenium 70-80 μm tall, with a granular brown epithecium, hypothecium pale. Interascal tissue of thick-walled paraphyses, irregular and slightly widened towards the apices. Asci 68-80 x 9-11 µm, cylindrical to cylindric-clavate, rather thick-walled, fissitunicate, the apex rounded, with a very well-developed apical cap that blues in iodine and an inconspicuous central canal, with a blue-staining outer mucous coat, 8-spored. Ascospores (10-)12-14(-15) × (4-) 4.5-6 μm, ellipsoidal to fusiform, the apices obtuse, aseptate, hyaline, thick- and smooth-walled, without an epispore or gelatinous sheath or appendages.
Chemistry: thallus C–, K–, KC+ yellowish, Pd–; medulla Pd± yellow, UV± (usnic, ± psoromic and ± 4-O-dimethylpsoromic acids). There are two chemotypes (medulla Pd+ yellow and Pd–; with or without psoromic acid), possibly with different ecology and distribution patterns in the British Isles. Morphs with a Pd+ yellow medulla (psoromic acid) are apparently confined to coastal sites whilst morphs with a Pd– medulla (without psoromic acid) also occur inland.
Assessed by Woods & Coppins (2012) as of Least Concern, but considered by Seaward (2009) to be decreasing in range especially in coastal habitats.
Mainly in N & W British Isles, primarily coastal but also inland in suitable habitats in the Pennines. BLS map here.
On soil, mosses and hard calcareous rocks, preferring crevices of limestone cliffs and pavements, also on calcareous dunes and serpentine.