Thallus: effuse and continuous to cracked or areolate, never distinctly effigurate at edge; prothallus: +present, white areoles: mostly contiguous but sometimes dispersed and +squamulose at margins, smooth to +rugose, thin, not marginally crenate to lobed surface: white to gray, green-white or pale brown, often tinged pink, K-, P- Apothecia: round, scattered; 0.2-0.8(-2) mm in diam. disc: rose-pink to red-brown or brown-black, +roughened, margin: thin, white, smooth to crenate, pseudothalline caused by its eruption through the thallus, part of which may remain adhering, +forming a halo-like rim, at least when young hymenium: hyaline below, hyaline to yellowish brown above, 90-140 µm tall; hypothecium: colorless ascospores: hyaline, simple, 14-25 x 7-13 µm Pycnidia: immersed conidia: filiform Spot tests: thallus K-, C+ reddish, KC+ reddish Secondary metabolites: thallus with gyrophoric acid chemosyndrome. Substrate and ecology: on siliceous rocks and stones, brickwork, occasionally on consolidated clay soil World distribution: cosmopolitan and widely distributed in temperate regions of both hemispheres Sonoran distribution: eastern Arizona and southern California Notes: Trapelia coarctata is characterized by a smooth thallus and pseudolecanorine apothecia. It is very variable. Immature apothecia may appear as scattered, starkly white, punctiform dots.