Thallus: foliose, small to medium-sized, up to 5 cm in diam., lobate lobes: flattened and elongate (up to 1 cm wide and up to 3 cm long); tips: rounded to subtruncate upper surface: gray to light brown, dull, bluish green when wet, thickly white-gray tomentose, sorediate soredia: granular, in orbicular, laminal soralia medulla: white, with +loosely interwoven hyphae photobiont: Nostoc lower surface: white, with anastomosing white or pale veins, rhizinate rhizines: white, flocculent, becoming darker and sparser centrally Apothecia: not seen in Sonoran material and rare elsewhere, up to 5 mm in diam. Pycnidia: not seen in Sonoran material Spot tests: C+ and KC+ red Secondary metabolites: methyl gyrophorate and gyrophoric acid in medulla (best seen in young soralia), rarely lacking. Substrate and ecology: on soil over and among mosses World distribution: temperate and boreal regions of North America, Europe, Asia and South America Sonoran distribution: infrequent in Arizona. Notes: Peltigera extenuata has only recently been recognized at species level distinct from P. didactyla. The latter species is often fertile, has darker veins and darker, less branched rhizines and lacks chemistry.