Thallus: foliose, approximately circular in outline (disintegrating into groups of lobes with age), medium sized, 5-10 cm in diam., adnate lobes: +flattened and elongate (1-1.5 cm wide and up to 4 cm long), often dichotomously branched, imbricate or separate; tips: rounded to subtruncate, often ascending and undulate upper surface: gray or blue-gray to brown when dry, blackish green when wet, smooth, dull to somewhat shiny, occasionally somewhat scabrose, sometimes marginally pruinose; without isidia; marginally sorediate soredia: pale gray to bluish tinged, coarsely granular medulla: white, with +loosely interwoven hyphae photobiont: Nostoc lower surface: white, with anastomosing pale, smooth, flattened veins, rhizinate rhizines: white, irregular, bushy or penicillately branched Apothecia: rare, +round to oblong, becoming saddle-shaped, semi-immersed on short, ascending lobes, up to 5 mm in diam. disc: dark brown to black, smooth ascospores: colorless to pale brown, acicular, 3(-5) septate, 35-65 x 4-5 µm Pycnidia: not seen in Sonoran material Spot tests: all negative Secondary metabolites: tenuiorin, methyl gyrophate, gyrophoric acid, hopane-6α,22-diol and unidentified terpenoids. Substrate and ecology: among mosses over bark (and occasionally rocks) in moist habitats at relatively intermediate to high elevations World distribution: temperate and boreal regions of North America, Europe and Asia Sonoran distribution: relatively common in Arizona, southern California, and Chihuahua. Notes: Peltigera collina is easily distinguished by its marginal linear soralia and etomentose thallus. The occasional occurrence on soil in Sonora is different from its ecology in many other areas, where this species is mostly found on mossy rocks and tree trunks in sheltered and moist habitats.