Thallus: areolate, indeterminate, sometimes over 3 cm wide areoles: angular, 0.3-1 mm in diam., relatively thin, 0.3-0.5 mm thick; rim: down-turned upper surface: white, dull, plane, smooth, when wetted yellow surface below becoming visible, with finely thick pruina algal layer: uniformly c. 100 µm thick medulla: white, prosoplectenchymatous, obscure in water, continuous with attaching hyphae lower surface: white, ecorticate, thin attachment: broad Apothecia: one or two per areole, immersed disc: black, round, 0.1-0.2 mm wide, plane parathecium: thin, c. 10 µm wide, often indistinct epihymenium: yellowish brown, conglutinated, c. 15 µm thick hymenium: hyaline, 100-110(-170) µm tall, paraphyses: 1.5-2 µm wide at base, apices barely expanded subhymenium: obscure, 10-15 µm thick hypothecium: obscure, thin asci: clavate, 70-80 x 12-15 µm, 100+-spored ascospores: hyaline, simple, broadly ellipsoid, 4-4.2 x 2-2.5 µm Pycnidia: not seen Spot tests: UV+ orange, all spot tests negative Secondary metabolites: rhizocarpic acid. Substrate and ecology: on limestone or granite World distribution: western North America (Organ Mountains, New Mexico) Sonoran distribution: rare in southern California (San Jacinto Mountains near Palm Springs). Notes: The description is based on the type (W!) and a collection by Wright at FH and will need revision once more specimens are available. Acarospora epilutescens is a yellow Acarospora and should not be confused with A. epilutescens sensu Hasse and Magnusson, which is treated within A. strigata (Knudsen 2005b).