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DescriptionLife habit: lichenized; Thallus: crustose, superficial, moderately thick or thin and immersed in substrate, continuous to rimose-cracked, fissured-areolate or warted; upper surface: gray, gray-green, or yellow; smooth or rugulose, dull or shiny; soredia or isidia: present or absent; upper cortex: present but often poorly developed, paraplectenchymatous in better developed specimens but often composed of little more than a thin, gelatinous epinecral layer; medulla: white; of irregularly interwoven hyphae, quite variable in thickness from almost neglible to quite thick in robuster specimens; photobiont: primary photobiont a Trebouxia, secondary photobiont absent; algal layer: horizontally continuous; algal cells 7-18 µm in diam.; lower cortex: absent; lower surface: not distinguishable from the substrate, attached by bundles of hyphae penetrating the substrate; Ascomata: apothecioid, either sessile with an expanded, open disc or almost closed and perithecial like and one to several immersed within a wart; disc: sometimes covered with granular soredia and appearing soralia-like or pruinose; hymenium: hyaline; hamathecium: lax, branched and richly anastomosing paraphysoids, 1.5-2.0 µm thick; asci: ± broadly cylindrical, apex with a broad ocular chamber, thick-walled, outer sheath K/I+ blue, otherwise K/I-, with an inner extensible layer, Pertusaria-type, 1-8-spored; ascospores: non-septate, thick walled, 1- or 2-layered, wall uniform or at times radiated canals to the surface, or ornamented; 60-250 x 20-80 µm; Conidiomata: pycnidial, rare; conidia: straight, acicular or bacilliform, 15-25 x 1-2 µm; Secondary metabolites: some combination of xanthones, ß-orcinol depsides, ß-orcinol depsidones, depsones or fatty acids; Geography: world-wide, particularly in subtropics and temperate regeions; Substrate: soil, moss, detritus, rocks or bark.; Notes: There is a stratification in the development of secondary metabolites; thiophanic acid and other xanthones, which partially screen incoming solar radiation, are best developed near the surface; the depsides, depsidones, depsones and fatty acids, in the medulla below the surface. Thus, in thick specimens the spot tests should be applied on a piece of the upper surface and on an exposed section of the medulla. The xanthones in general react C+ orange and are strongly fluorescent (yellow to orange) in long-wave UV. Specimens can thus be rapidly screened for xanthones with UV light with the caution that specimens with low xanthone concentrations will not necessarily be fluorescent. In short, a UV- specimen may still have low xanthone concentrations (e.g. P. consocians) whereas a UV+ specimen always has xanthones. Because many specimens have very thin medullas, it is not practical to differentiate upper surface and medullary reactions among the spot test given with each species description below. In general the spot test in Pertusaria occur in both the cortex and medulla with the exception of the xanthones (C+) that are often restricted to the cortex. Thus the descriptions do not distinguish between cortex and medulla. Brown spored species, sometimes treated as the genus Melanaria, will be treated in vol. II.