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, up to several cm across, sometimes appearing areolate; squamules: irregular in shape and with crenate to digitate margins, scattered to confluent; margin: not effigurate nor with elongated, stellate lobules upper surface: dark olive, brownish to black, often isidiate lower surface: dark, thallus resting on a conspicuous hypothallus surrounded by a blue prothallus Apothecia: semi-immersed to sessile, laminal, lecideine, 0.5-1 mm wide; disc: dark brown to black, open, concave, flat to slightly convex; exciple: thick, dark violaceous to greenish, often shiny, becoming flexuose; epihymenium: dark violaceous or dark greenish; hymenium: 70-115 (-175) µm high, amyloid; paraphyses: distinctly septate, sparingly branched, apical cells pointed or thickened; subhymenium: brownish asci: 8-spored, sometimes fewer ascospores: 2 to 4-celled, hyaline, narrowly ellipsoid, (7-) 10-17 (-22) x 3.5-5.5 (-6) µm; walls: thin Pycnidia: dark colored, immersed, laminal, up to 0.1 mm wide conidia: slightly dumbbell shaped, hyaline, 3.5-5 x 1 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on various calcareous rocks, mainly limestone, rarely on limy siliceous rock, soil or bark World distribution: widely distributed in the Northern Hemisphere Sonoran distribution: common in relatively moist habitats in Arizona, southern California and Chihuahua. Notes: This variable species differs from all other Sonoran Placynthium species in the lack of elongated marginal squamules and a non-stellate growth form. A blue prothallus may be also present in Placynthium asperellum. Some of the Sonoran samples of Placynthium nigrum can be referred to as either P. nigrum var. tantaleum or P. tantaleum on acount of slightly broader (6-8 µm), (1-) 2-celled spores and a less distinct blue prothallus.