Thallus ± orbicular, squamulose (squamules 0.1-0.5 mm diam.) to ± placodioid, thick, puckered and ± folded in ridges, lobed at the margins, bright yellow-green, occasionally yellow and sometimes stained brownish in parts, the surface interspersed with yellowish crystals. Medulla well-developed, not blueing in iodine, dark brown beneath, with rhizine-like hyphal strands extending into the substratum. Prothallus absent. Photobiont unidentified coccoid green algae.
Anamorph: not known.
Teleomorph: ascomata apothecia, to 2 mm diam., initially immersed, arising between the squamules, with a thin crenulate margin, the cortical tissues surrounding the ascomata dark red in section. True exciple soon excluded, dark reddish brown, composed of cells with scarcely distinct lumina. Thalline exciple absent. Hymenium 100-120 μm tall. Hypothecium dark brown, reddish in parts. Interascal tissue composed of thin-walled frequently branching and anastomosing paraphyses 2-3 µm in diam. immersed in a gelatinous matrix, the apical cells slightly inflated, within a dark olive-green to ± black epithecium that is, K–, N+ rose. Asci 50-70 × 14-17 μm, cylindric-clavate, the outer coat K/I+ blue, ± thickened at the apex, with an internal deep blue K/I+ meniscus-like cap, 8-spored. Ascospores (12-)13-17(-18) × 7-10 μm, ellipsoidal, rounded or occasionally broadly apiculate at the ends, initially grey to olivaceous and maturing dark brown, 0- to 1- (to 3-) septate, smooth, thick-walled, the slightly constricted central septum thickened with an internal torus, lacking a distinct perispore.
Chemistry: thallus C–, K–, KC–, Pd+ yellow-orange (rhizocarpic acid and perhaps argopsin).
Assessed as Vulnerable (D1) by Woods & Coppins (2012). It is considered to be Nationally Rare, and is listed in Schedule 8 of the Wildlife and Countryside Act 1981 and in Section 2(4) of the Nature Conservation (Scotland) Act 2004.
Separated from Epilichen scabrosus by its better-developed thallus with apothecia forming between rather than on the squamules. Species of Arthrorhaphis such as A. alpina and A. citrinella have similar coloured thalli and occupy broadly similar montane habitats, but have matt rather than glossy squamules and a Pd- thallus medulla.
In GB&I restricted to high montane regions of Scotland, where it is centred in the Ben Nevis Range with outlying populations in Glen Coe, on Ben Alder, the Cairngorms and on basalt and gabbro on the Isle of Skye.
On acid solifluction soils, the vertical side of large boulders, mosses and rock crevices above 900 m.