Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: small-squamulose, resting on a thin blackish hypothallus that contributes to the dark color of this species squamules: rarely more than 1 mm wide, incised, upper surface: gray-blue or brown, isidiate but not sorediate isidia: coralloid, digitate but occasionally granular, marginal, sometimes obscuring the squamules upper cortex: paraplechtenchymatous, 25-35 µm thick Apothecia: uncommon, up to 1 mm diam, biatorine; disc: red-brown, often becoming blackish and convex; exciple: paraplectenchymatous, up to 90 µm thick; hymenium: primarily hyaline but brown-black above, I+ persistently deep blue,up to 100 µm high; subhymenium: thick, brownish asci: clavate to subcylindrical, with apical amyloid tubes, 8-spored ascospores: simple, colorless, ellipsoid, without an epispore, 10-17 x 5-8 µm Pycnidia: not seen Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on sheltered, wet montane rocks at fairly high altitudes (2000-3000 m); elsewhere also on bark World distribution: cool-temperate, widespread in the Northern Hemisphere Sonoran distribution: SE Arizona and adjacent Sierra Madre Occidental of Chihuahua and Sonora. Notes: This species is quite distinctive from P. clavulifera, in being smaller, more crustose, and darker with finer isidia. Sometimes flattened lobules are found inbetween the isidia, but the thalli are never just lobulate. Two saxicolous specimens now included in this species were provisionally identified as P. saxicola P. M. Jørg. but are now included in this species. Poorly isidiate specimens do resemble P. saxicola, a species from northern Venezuela that lacks isidia and has characteristically curved, central squamules.