Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: repeatedly squamulose, occasionally with radiating lobes on smooth bark, forming irregular rosettes up to 5 cm diam., resting on a blackish, variously developed, but mainly inconspicuous hypothallus squamules: up to 3 mm wide and 100 µm thick, leafy, often imbricate upper surface: gray-blue to brownish, lobulate, lacking isidia or soredia lobules: usually gray-blue, occasionally brownish upper cortex: paraplectenchymatous, up to 30 µm and at least two cells thick, Apothecia: numerous, up to 1.5 mm diam.; disc: reddish brown; proper margin: distinctly paler, paraplectenchymatous, up to 150 µm thick; hymenium: I+ blue, shallow, up to 90 µm high asci: clavate, with distinct amyloid apical tubes, 8-spored ascospores: simple, colorless, smooth, ellipsoid, 11-17 x 6-8 (-10) µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on deciduous trees, mainly oaks in mist forests about 2000 m World distribution: widespread in subtropical parts of South and Central America, the West Indies, Macaronesia, and Japan Sonoran distribution: scattered in the subtropical moist, montane forest of the Sierra Madre Occidental in Sinaloa. Notes: Previously it was confused with the much more crustose and southern P. nigrocincta (Mont.) Müll. Arg., from which it is easily distinguished by its repeatedly squamulose, imbricate, non-waxy lobes. In contrast, P. nigrocincta has adpressed, waxy squamules. Also P. clavulifera has smaller spores than P. nigrocincta.