Thallus: rimose-areolate, indeterminate, continuous, overall up to 4 cm wide areoles: angular, contiguous, rarely subeffigurate with some shallow irregular lobes on edge (in one high elevation specimen), usually 0.2-0.5(-1) mm wide, often very thin, less than 0.4 µm thick; rim: lacking upper surface: yellow, plane, smooth, dull, epruinose cortex: paraplectenchymatous, 25-40 µm thick; syncortex: thin; eucortex: opaque yellow above; hyaline below algal layer: dense to thin, with uneven upper and lower borders medulla: white, prosoplectenchymatous attachment: broad, without a stipe Apothecia: one or more per areole, immersed, below or even with upper surface, 0.1-0.3 mm in diam. disc: blackish to reddish brown, concave to plain, round to irregular in shape, often with interascal sterile plectenchyma that is eventually dissolved, occasionally with a thalline collar pushed up by expansion of the apothecium parathecium: narrow to wide, c. 50 µm thick, but can expand to form parathecial crown epihymenium: dark brownish yellow, 15-30 µm thick hymenium: hyaline to pale yellow, 85-125 µm tall; paraphyses: coherent, 1.6-1.9 µm in diam. at base, apices slightly expanded subhymenium: hyaline, 25-30 µm thick; hypothecium: hyaline, 50-100 µm thick asci: clavate, 60-80 x 16-23 µm, 100+-spored ascospores: hyaline, simple, narrowly ellipsoid, 4-5 x 1.7-1.9 µm Pycnidia: not seen Spot tests: UV+ orange, all spot tests negative Secondary metabolites: rhizocarpic acid. Substrate and ecology: on acid rocks from 500 meters in elevation to c. 3350 m World distribution: western North America (Arizona to Texas), Mexico Notes: Acarospora contigua is easily identified by its epruinose, rimose-areolate thallus, but it can be confused with other species, like A. socialis, which have an areolate stage in their development, or with the effigurate species when the edge is missing. A Darrow collection (ASU) from the San Francisco Peaks developed as a subsquamulose thallus with subeffigurate margins when coming into contact with other crustose lichens; otherwise, on a smooth surfaces, it developed without an effigurate margin. Few specimens were seen during this study and this species needs a more extensive investigation. It is a yellow equivalent of A. scotica. Golubkova's (1983) use of A. contigua, reflected in Lichens of North America (Brodo et al. 2001) with Roux determining an Acarospora species from Ozarks as A. contigua, is rejected in this treatment as not being consistent with the type. Some of her work does not transcend Magnusson's treatments and adopts too literal a dependence on Magnusson's keys and descriptions.