Thallus epilithic, areolate to subsquamulose; areoles scattered to contiguous, fertile areoles (0.6–) 1.0–2.4 mm diam, sometimes with large squamules to 4.5 mm diam; upper surface brown-grey to brown, more rarely whitish or brownish black, flat to uneven and strongly convex, dull to somewhat shiny, sometimes with concentric, wavy markings, epruinose. Epinecral layer present, (2–)5–30(–45) µm thick, often very uneven in thickness. Upper cortex 30–60 µm thick, with crystals in usually large, scattered groups, sometimes rather sparse.
Anamorph: Pycnidia rare; conidia ellipsoidal ca 2 × 1 µm in size.
Teleomorph: Ascomata apothecial, 1–4(–14) per areole, immersed; disc 0.15–0.50(–0.70) mm diam. often dilated, sunken or level with the thallus, brown to dark brown, smooth to very uneven, epruinose. Proper exciple colourless, c. 30 µm thick below the hymenium, widening to ca 90 µm in the upper part; epihymenium reddish brown to golden brown to dark brown; hymenium 120–230 µm tall. Interascal tissue of paraphyses, 1– 1.5 µm diam, often branched near the tips, the tips ±cylindrical or weakly clavate, to 2 µm diam. Subhymenium colourless to yellowish or greyish, with much oil, 70–90 µm tall. Asci to 120 × 22 µm. Ascospores 3–5 × 1–1.5 µm, narrowly ellipsoidal, hyaline, aseptate.
Similar to M. smaragdula, but with larger, much thicker and strongly convex areoles or squamules. The apothecia are often dilated and then have a strongly uneven disc.
In GB&I, reported from England (Devon) and Wales (Cardiganshire).
On siliceous rocks (granite and gritstone) in upland areas.