Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.
Thallus: foliose, approximately circular in outline, medium to large, 5-10 cm in diam., adnate lobes: +flattened and elongate (1-2 cm wide and up to 4 cm long), imbricate or separate; tips: rounded to subtruncate, sometimes undulate upper surface: gray, or blue-gray, often tinged brown when dry, blackish green when wet, smooth, shiny, without isidia and soredia medulla: white, +loosely interwoven hyphae photobiont: Nostoc lower surface: white, with anastomosing, smooth, flattened veins becoming darker centrally, rhizinate rhizines: brown to black, fasciculate, in concentric circles Apothecia: not seen in Sonoran material, elsewhere +round, on short, ascending lobes, up to 7 mm in diam. disc: flat, dark brown to black ascospores: colorless to pale brown, acicular, 3(-5) septate, 30-50 x 4-7 µm Pycnidia: not seen Spot tests: all negative Secondary metabolites: tenuiorin, methyl gyrophate, gyrophoric acid, hopane-6α,22-diol, hopane-6α,7α,22-triol and unidentified terpenoids. Substrate and ecology: among mosses over soil at high elevations World distribution: temperate and boreal regions of North America, Europe and Asia Sonoran distribution: infrequent in Arizona (Coconino). Notes: If fertile, P. horizontalis is easy to distinguish by its flat, round apothecia and glossy thallus. When sterile, the thick, blunt rhizines growing in +concentric lines are helpful in identifying it, and its chemistry is different from other species, except P. elisabethae, which has schizidiate, veinless thalli.