Thallus: areolate or subsquamulose, overall up to 5 cm wide areoles: angular up to 1.8 mm in diam., up to 0.7 mm thick, dispersed to contiguous, indeterminate but outer areoles of small groups sometimes with extended lobes and appearing determinate; rim: down-turned, corticate, concolorous with thallus upper surface: yellow, dull, plane to convex, smooth to rough, epruinose or pruinose lateral cortices: paraplectenchymatous, 30-60 µm thick, lacking a syncortex; eucortex: yellow above, obscurely white below algal layer: thin, continuous, slightly uneven, scattered beneath apothecium medulla: white, prosoplectenchymatous, thick, continuous with attaching hyphae attachment: broad, thick, eventually elevating the areole (gomphate), without a stipe lower surface: outer edge yellow and corticate, inner edge white to pale brown, ecorticate Apothecia: 1-5 per thallus unit, immersed, punctiform to dilate, sometimes lecanorine with thallus forming rugulose margin disc: reddish brown, round to irregular, sometimes with interascal plectenchyma parathecium: 20-30 µm thick epihymenium: yellow, weakly conglutinate, 10-15 µm thick hymenium: pale yellow above to hyaline below, 80-110 µm tall; paraphyses: 1-1.8 µm in diam. below, spreading in water, without expaned apices or dark caps subhymenium: pale yellow, up to 40 µm thick; hypothecium: 20-30 µm thick asci: clavate, height varying, 10-17 µm wide, 100+-spored ascospores: hyaline, simple, ellipsoid, c. 4 x c. 2 µm Pycnidia: spherical, c. 100 µm in diam., ostiole visible as light brown dots conidia: bacilliform, 3-4 x 1(-1.5) µm Spot tests: UV+ orange, medulla K+ red, often forming crystals Secondary metabolites: rhizocarpic acid, norstictic acid (sometimes at low levels). Substrate and ecology: on acidic and volcanic rocks in full sun, often growing with Lecanora valesiaca World distribution: Asia, Europe, and North America Sonoran distribution: Arizona. Note: Acarospora heufleriana is probably undercollected but appears to occur locally in North America. Acarospora rubicunda is a specimen with pruinose apothecia.