Thallus: foliose, large, 5-15 cm in diam., adnate lobes: +flattened and elongate (1-3 cm wide and up to 4 cm long), imbricate or separate; tips: rounded to subtruncate, often undulate upper surface: gray or blue-gray to brown when dry, bright green when wet, smooth, dull to somewhat shiny, marginally tomentose medulla: white, with +loosely interwoven hyphae photobiont: primary one Coccomyxa and with convex laminal cephalodia containing Nostoc lower surface: white, with anastomosing pale, smooth, flattened veins, rhizinate rhizines: white, irregular, bushy or penicillately branched Apothecia: +round to oblong, becoming saddle-shaped, semi-immersed on short, ascending lobes, up to 9 mm in diam.; under side of apothecia: with green corticate patches; margin: smooth to crenulate disc: flat, dark brown to black, smooth ascospores: colorless to pale brown, acicular, 3(-5) septate, 65-85 x 4-5 µm Pycnidia: not seen in Sonoran material Spot tests: all negative Secondary metabolites: tenuiorin, methyl gyrophate, 2-4 unidentified terpenoids. Substrate and ecology: among mosses over soil in moist habitats at relatively high elevations World distribution: temperate and boreal regions of North America, Europe and Asia Sonoran distribution: infrequent in Arizona. Notes: Peltigera leucophlebia has often been mistaken for the allied P. aphthosa, a species not yet reliably recorded from the Sonoran area but potentially found on sterile mossy soils at higher elevations. It has a thick thallus, is non-veined underneath and has predominantly flattened cephalodia. The under side of the apothecia is continuously corticate, whereas in P. leucophlebia only small green cortical patches are found.