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DescriptionLife habit: lichenized, rarely lichenicolous; Thallus: crustose, verruculose, areolate, squamulose or placodioid; areoles: dispersed or rimose-areolate, indeterminate or determinate; or sometimes becoming elevated with expansion of the mycelial base above the substrate ("gomphate") and subsquamulose, or beginning as areoles and developing stipes (usually half the diameter of areole) and becoming squamulose, or determinate and developing an effigurate margin with an areolate or subsquamulose center; rim: usually down-turned; upper surface: pale to medium or blackish brown or yellow (with rhizocarpic acid), dull or glossy; smooth to rugulose, epruinose or pruinose (then often appearing white), often developing fissures, rarely faveolate (A. obpallens); upper cortex: generally differentiated into three layers: a syncortex composed of an upper gelatinized epicortex [lacking or up to 100 µm thick] or sometimes replaced by an epinecral layer (e.g. A. nodulosa), and paraplectenchymatous [rarely prosoplectenchymatous] eucortex made up of an upper pigmented layer, and a lower, usually hyaline layer, rarely inspersed; lateral cortex: generally continuous with upper cortex, sometimes thinning, sometimes forming a rim with the same color as upper cortex or darker; attachment: always broadly attached in young areoles, remaining so in some species, or mycelial base thickening above the substrate (elevating the areole which usually becomes subsquamulose) or developing a stipe, or attached by rhizohyphae and pseudorhizines; lower surface: lacking or narrow around attachment interface, ecorticate, subcorticate, or eucorticate, pigmented or white, or often discolored by substrate or sediment; lower cortex: ±continuous with upper and lateral cortices and usually paraplectenchymatous or appearing superficially paraplectenchymatous but formed by conglutinated apices of medullary prosoplectenchyma; photobiont: primary one a chlorococcoid green alga, secondary one absent; cells 8-15 µm in diam.; algal layer: either forming an even stratum or forming an uneven layer penetrated by anticlinal bundles of hyphae; medulla: generally white, prosoplectenchymatous in Sonoran species, intricate, hyphae often obscured, usually inspersed, continuous with attaching hyphae, often also containing mineral particles from substrate; Apothecia: usually immersed, aspicilioid, one or more per areole, verruca or squamule, sometimes expanding and forming a thalline margin (A. badiofusca), or occasionally elevated above primary thallus with a thalline margin (A. nodulosa, A. interspersa) or a margin formed from true exciple (A. scabrida); disc: usually round but sometimes compressed or irregular, black to brown to red or yellow, smooth or rough, with or without interascal formations of sterile plectenchyma, epruinose or pruinose; true exciple: usually distinct, prosoplectenchymatous; @parathecium: 10-20 µm thick but sometimes expanding up to c. 70 µm around the surface of an apothecium, sometimes hardening and becoming pigmented, forming a ring around the disc (a parathecial crown); epihymenium: usually conglutinate in a pigmented gel, generally dissolving in K; hymenium: generally hyaline, not inspersed, 70-200 µm tall, (including epihymenium); paraphyses: usually not branched, 0.5-3.5 µm in diam. at base, usually septate, with or without oil drops, septa sometimes constricted in upper third, apices not expanded or expanded to up to 5 µm, sometimes in pigment hood or gel cap, usually conglutinate; subhymenium: hyaline or pale yellow, obscure, 10-50 µm thick; hypothecium: generally hyaline or pale yellow, indistinct or 10-50 µm thick; asci: narrow to clavate, sometimes becoming inflated, usually shorter than epihymenium but sometimes protruding into epihymenium, effectively unitunicate, outer wall K/I+ blue, tholus K/I-; dissolving when spores are mature; polyspored, (16-)100+-spored; ascospores: hyaline, simple, globose to ellipsoid, occasionally containing oil drops, with or without a distinct perispore or mucilaginous sheath; Conidiomata: pycnidial, usually spherical with a hyaline exciple, not observed in many specimens; conidia: hyaline, usually very short bacilliform, 2-3 x 1-2 µm; Secondary metabolites: often none, or sometimes with norstictic, gyrophoric, or fatty acids; rhizocarpic acid present in yellow species; Substrate: on soil or acid or basic rocks, other lichens, often nitrophilous; Geography: cosmopolitan but most common in open, arid habitats.; Notes: Acarospora heppii has been transferred to Myriospora (Harris and Knudsen 2006). Although Reeb et al. (2004) concluded that the genus Acarospora is not monophyletic, there is insufficient morphological, chemical and molecular data to propose an alternative classification at this time. This treatment agrees with Reeb et al. (2004) in rejecting the traditional subgenera Xanthothallia and Phaeothallia. Pleopsidium is provisionally accepted but the concepts underlying the separation of flavum and chlorophanum from Acarospora such as paraplectenchymatous cortex and chemistry as well as its placement in a different order are rejected. The use of a yellow or brown section in the keys is artificial and does not represent the phylogeny of the genus. Magnusson is generally considered a "splitter" and most of synonyms made in this treatment are made because the author thinks Magnusson was not aware of the full range of variation in many species because he had too few specimens available (Knudsen 2004). The different species concepts delimited here will eventually need testing by molecular investigations, but much basic taxonomic and ecological investigation is needed first. Many species in this treatment are known from only a few specimens, such as A. brattiae, A. calcarea, A. contigua, A. epilutescens, A. hassei, A. nevadensis, A. novomexcana, and A. wetmorei. Cortical measurements include syncortex and eucortex together. The cortex is generally paraplectenchymatous, but, especially in species with jagged algal layers, there are prosoplectenchymatous areas. Variation in cortical thickness has led to rejection of Magnusson's over-emphasis of thickness as a character. In many species, the proper exciple forms a ring around the immersed disc and may be mistaken for a thalline margin; in this treatment such a ring is called a parathecial crown. Occasionally thalli are sufficiently eroded that the true exciple is exposed and becomes black. Sometimes the apothecium develops later in a mature thallus by dissolving sterile thallus plectenchyma and often leaving externally an umbo or gyrose interascal plectenchyma (esp. A. peliscypha and A. socialis). Often several apothecia will merge leaving an umbo or peninsula of sterile plectenchyma on at least the surface of the apothecia. A mucilaginous sheath can sometimes be seen around mature spores discharged from asci among paraphyses, but it dissolves in water or K or disappears in old specimens; nonetheless, a few species typically have a noticeable perispore. Generally when the thalli are referred to as being areolate or squamulose, this refers to stages in the ontogeny of the individual units from being broadly attached to forming a stipe and becoming elevated above the substrate. Subsquamulose refers to species that are elevated above the surface of substrate through thickening of attaching mycelial base (gomphate) or are broadly attached but developing some lobes, a variable character. Sometimes hyphal bands penetrate the algal layer, causing it to appear uneven, and this pattern is an informative character in some species, but occasionally this pattern is obscured in a vertical section and is best seen with a horizontal section of the thallus. Descriptions are based on types when available and Sonoran specimens, and additional specimens were also consulted from the rest of North America, South America, and Europe.