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DescriptionHabit: lichenized; Thallus: usually foliose, but also crustose to squamulose to dwarf fruticose, + gelatinous, usually lobate; lobes: sublinear to subirregular or terete, elongate or short, +plane, separate or occasionally imbricate, (0.1-) 3-15(-20) mm wide; apices: rotund to oblong, entire to crenate or irregularly dissected, sometimes upturned or becoming erect in a few species; upper surface: often medium to dark gray, rarely bluish gray, sometimes brown to occasionally blackish brown, dull or shiny, smooth or often strongly wrinkled, sometimes with isidia, phyllidia or lobules but soredia and pseudocyphellae absent; photobiont: primary one a species of Nostoc, secondary one absent; internal anatomy: typically with both upper and lower cortices consisting of a single (rarely double) layer of irregularly isodiametrical cells, internally usually homiomerous between the cortices but paraplectenchymatous in one group, usually with loosely interwoven chains of Nostoc and hyphae; lower surface: same color as the upper surface but usually lighter, smooth or wrinkled, sometimes with a dense white tomentum of cylindrical or spherical hairs or otherwise with scattered tufts of rhizines or hapters; Ascomata: apothecial, +present, discoid, laminal to occasionally marginal, sessile to stipitate (very rarely immersed); disc: usually brown or red-brown, concave to plane to convex; margin: usually thalline, concolorous with the thallus or darker, smooth to rugulose or wrinkled, rarely pubescent, often isidiate or lobulate in species having those structures; exciple: cupular, often forming a raised ring with a contrasting color to the thalline margin, euparaplectenchymatous, sometimes poorly developed centrally; hymenium: hyaline below and pale brown or yellow above ("epihymenium"), 50-250 µm tall; paraphyses: usually unbranched, conglutinate, 1-2 µm wide, usually slightly inflated apically; subhymenium: hyaline to pale yellow; asci: cylindrico-clavate, (4-)8-spored; ascospores: hyaline, usually submuriform or muriform, more rarely only septate transversely, ellipsoid or ovoid to subfusiform, often with acute ends and rarely acuminate ends, 15-50 x 6-18 µm; Conidiomata: pycnidial, endo- or exobasidial, rarely observed; conidia: hyaline, simple, bacilliform; Spot tests: all negative; Secondary metabolites: none detected; Geography: cosmopolitan, from the tropics to polar regions; Substrate: on bark or acidic or basic rocks, more rarely on soil, often among mosses.; Notes: Usually Leptogium is easily distinguished from Collema, that has a more olivaceous to black, pulpy thallus without a proper cortex. The two genera are, however, not always clearly differentiated, and several of the sections accepted in Leptogium appears to be more closely related to Collema species than to other Leptogium species. Because Leptogium, unlike Collema, has not been monographed on world level, this question has not been fully resolved. The small species of Leptogium are particularly in need of further investigation in both the Sonoran region and worldwide. Leptochidium albociliatum is often mistaken for a Leptogium, though it has characteristic white hairs marginally, a poorly developed cortex, and one-septate spores.