Thallus: areolate to rimose-areolate, squamulose or verruculose, overall up to 4 cm wide areoles: round to angular, sometimes lobed, 0.2-0.5(-1.5) mm in diam., less than 0.6 mm thick, dispersed or contiguous; rim: down-turned, often a darker brown to black upper surface: usually medium to dark brown, seldom light brown, sometimes black, dull, plane to convex, smooth or rarely fissured, epruinose to heavily pruinose upper cortex: paraplectenchymatous, thin, 10-20(-55) µm thick; cells: distinct in K, round to rectangular, (1-)2-3(-5.2) µm wide or 4-9 x 4 µm; syncortex: lacking or up to 20 µm thick; eucortex: upper layer dark brown and c. 10 µm thick, lower layer hyaline algal layer: even medulla: white, subparaplectenchymatous to prosoplectenchymatous, continuous with attaching plectenchyma, distinct in K lower surface: narrow, corticated, cells one to several layers thick, usually white, sometimes dark or light brown, or light green from algal layer; sometimes raised above substrate layer at least 10 µm by stipe, allowing soil particles to collect under squamule, sometimes giving edges of scattered squamules a fuzzy appearance at 10 X attachment: broad to rarely forming a stipe Apothecia: deeply immersed, rounded to angular, one to several per areole, 0.1-0.4(-1) mm in diam.; cortex between apothecia usually without fissures; sometimes apothecia merging leaving sterile remnants as an umbo or peninsula disc: dull reddish brown to blackish brown, plane to convex, smooth to rough parathecium: 10-20µm wide, sometimes merging cortex overlapping disc epihymenium: light yellow or reddish brown to dark brown; sometimes with dark pigment caps, 10-20 µm thick hymenium: hyaline, ±110 µm tall; paraphyses: 1.3-2.1 µm wide at base, spreading (in K); apices 3.3-4.1 µm wide; rarely bifurcating in upper third subhymenium: hyaline or pale yellow, indistinct, 10-15 µm thick; hypothecium: 10-50 µm thick asci: cylindrical, 75-90 x 15-23 µm, 100+-spored ascospores: hyaline, simple, narrowly to broadly ellipsoid, (3-)3.8-5 x 1-2.1 µm Spot tests: UV-, all spot tests negative Secondary metabolites: none detected. Substrate and ecology: on acidic rocks and basalt, occasionally on wood World distribution: Northern Hemisphere Sonoran distribution: common in Arizona, southern California, Baja California, Baja California Sur, Sonora, and Durango (just outside of the Sonoran region). Notes: Acarospora veronensis is interpreted as an extremely variable species, and it is one of the most common species in the Sonoran region. Acarospora americana was applied by Magnusson to specimens usually forming a rimose-areolate crust. Acarospora cinereoalba (Fink) H. Magn. and A. superfusa H. Magn. were applied to specimens that are heavily pruinose, sometimes forming verrucae with a single disc. Such specimens are common in arid areas of southern California, and in the deserts of Arizona and Mexico, but similar specimens have been seen collected from near waterfalls in Minnesota (Fink, MIN) and from New York (Wetmore, MIN). Some of these heavily pruinose specimens can only be distinguished from similar, non-faveolate specimens of A. obpallens by the C test or TLC. Sometimes they look like A. strigata, which always has wider spores. As with several other highly polymorphic Acarospora taxa, combined molecular, morphological and ecological studies may eventually segregate new species from this species concept. As with yellow Acarospora, the narrow taxonomic concepts of Magnusson are avoided (Knudsen 2004).