Sonoran distibution: Arizona (known from a single collection from Tonto National Forest at 525 meters, Schroeder 454, ASU!) Sonoran area: rare in Arizona and Sonora. squamules: 0.5-2(-4) mm across, 0.2-0.7 mm thick, irregular, angular or rounded, sometimes crenulate or lobed, not imbricate or crowded, except in center of very old thalli, vegetatively dividing; on soil, areoles discretely surrounded by distinct outline in soil caused by development of rhizohyphal mass; rim: down-turned or undulate, when crowded or squamulose and sometimes up-turned lower cortices: continuous with upper cortex, thinning below Thallus: areolate or squamulose (concave or flat with lobes), indeterminate, overall up to 10 cm wide areoles: round, angular, 0.4-1(-2) mm wide, 1 mm thick, discrete to contiguous or imbricate, interconnected by dense rhizohyphae; rim: ±down-turned upper surface: yellow to bleached yellow, dull, rough, uneven, usually with multiple creases, epruinose or pruinose cortex: prosoplectenchymatous to paraplectenchymatous, syncortex and eucortex not easily delimited, 20-70 µm thick, greenish yellow algal layer: thickness various, even to uneven, penetrated by hyphal bundles medulla: white when dry, sometimes pale yellow from diffusing rhizocarpic acid in mounts of water or K, prosoplectenchymatous, often with small cavities, obscure in water due to granules, continuous with attaching rhizohyphae lower surface: yellow, upper cortex continuing beneath rim of squamulose areoles, thinning out attachment: broad with abundant anastomosing rhizohyphae, and without forming a stipe rhizohyphae: 3-5 µm thick, dense, anastomosing, often gelatinizing together around soil particles, firmly anchoring the thallus to the earth rhizines: pseudorhizinate, often 0.2 mm thick, formed of gelatinized rhizohyphae, penetrating deeper into the substrate Apothecia: usually one per areole, but sometimes 2-3 disc: black to reddish brown, 0.4-1.2 mm in diam., epruinose or slightly pruinose, immersed, expanding to fill whole areole, forming a lecanorine apothecium, or sessile with a parathecial crown and/or thalline margin parathecium: 10-50 µm wide, sometimes forming a parathecial crown epihymenium: red- to-yellow-brown, conglutinated, c. 15 µm thick hymenium: hyaline, 85-130 µm tall; paraphyses conglutinated, 1.7-2.1 µm in diam. at base, apices barely expanded subhymenium: hyaline to pale yellow, 30-100 µm thick; hypothecium: up to 20 µm thick asci: narrowly to broadly clavate, height varying with height of the hymenium, 100+-spored ascospores: hyaline, simple, globose to broadly ellipsoid, 2-4 x 2-2.5 µm Pycnidia: not seen Spot tests: UV+ orange, all spot tests negative Secondary metabolite: rhizocarpic acid. Substrate and ecology: on soil World Distribution: Europe, Africa, and North America Sonoran distribution: southern California, and Baja California. Notes: Acarospora schleicheri becomes rare when weeds invade soil habitats. The primary species with which A. schleicheri can be confused is A. rhabarbarina, which has mostly non-imbricate areoles except in very old specimens. In addition, A. socialis rarely occurs on consolidated soil, where it seldom forms a stipe and is non-imbricate too, and specimens of A. socialis usually grow on rocks in the immediate area. The name of this species has been erroneously applied to dozens of other yellow species worldwide.