Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.
Thallus: foliose, small to medium-sized, 1-5 cm in diam., often in small coalescing groups lobes: +flattened and elongate (up to 1 cm wide and 2-3 cm long), imbricate or separate; tips: rounded to subtruncate, strongly ascending upper surface: gray or whitish to bluish gray when dry, blackish green when wet, smooth, dull, thin white-gray tomentose, without isidia, when young sorediate soredia: laminal, granular, in round to irregular, ulcerose, whitish soralia (up to 2.5 mm wide) that later become confluent, disappearing with the formation of apothecia medulla: white, with +loosely interwoven hyphae photobiont: Nostoc lower surface: pale tan to cream-colored, with anastomosing pale, ochraceous, raised veins, rhizinate rhizines: white, simple or bottle-brush-like Apothecia: frequent, oblong, saddle-shaped, marginal and c. vertically oriented, up to 5 mm in diam. disc: flat, red-brown to brown, smooth ascospores: colorless to pale brown, very narrowly fusiform to acicular, 3-7 septate, 55-75 x 3.5-4.5 µm Pycnidia: not seen in Sonoran material Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: among mosses over soil World distribution: temperate and boreal regions of North America, Europe and Asia Sonoran distribution: occasional in Arizona, southern California and Baja California. Notes: Laminal soralia and tomentose thallus distinguish P. didactyla from other species of the genus except P. extenuata, which has whitish lower surface and white floccose rhizines as well as having medullary substances.