Life habit: lichenized Thallus: crustose, areolate to rimose-areolate, margin thinning at edge; without elongated lobes; prothallus: absent surface: gray to olive gray, smooth, without asexual propagules cortex: cellular, thin Apothecia: adnate, 0.5-1 mm in diam., lecanorine disc: brown, flat, epruinose margin: persistent, slightly raised; thalline margin present, concolorous with thallus; proper margin not visible parathecium: cellular (paraplectenchymatous); exciple below hypothecium amorphous epihymenium: brown, K+ violet, H+ violet, 10%N + violet, cN+ violet, C+ violet hymenium: hyaline, 70-100 µm tall paraphyses: 2-3 tip cells swollen, not branched or with few branches; subhymenium hyaline asci: cylindrical, 8-spored ascospores: hyaline, 2 locules, ellipsoid, 14-17 x 7-8.5 µm, isthmus 1.5-3 µm, spore end wall thin Pycnidia: not observed Spot tests: apothecial margin K+ violet, H+ violet, 10% N+ violet, cN+ violet, C+ violet; thallus K+ violet, H+ violet, 10%N+ violet, cN+ violet, C+ violet Secondary metabolites: unidentified anthraquinones and thalloidima green. Substrate and ecology: on non-calcareous or calcareous rocks World distribution: North America Sonoran distribution: southern California and Arizona. Notes: Caloplaca atroalba has non-stipitate areolate thallus, lack of pruina, thin thallus cortex, and an even algal layer. Some thalli on rough rock are areolate to the margins. On soft sandstone the thallus is irregular and thin and approaches C. oblongula but that species has a lecideine apothecial margin. Caloplaca atroalba may have a thin necral layer on some thalli or, in some cases on soft sandstone, the upper surface is a powdery layer. The thallus is usually thinner than in C. albovariegata. Caloplaca variabilis has pruinose discs or apothecial margins and the dense fine granules are insoluble in KOH.