Phytologia 28: 335 (1974).

Basionym: Parmelia arnoldii Du Rietz, Nytt. Mag. Natur. 62: 80 (1924).

Thallus: foliose, adnate to loosely adnate, 3-20 cm in diam., lobate lobes: subirregular, elongate, slightly imbricate, plane, separate, 10-20 mm wide; apices: rotund, often ascending and wavy, ciliate; cilia: up to 5 mm long upper surface: gray with some blackened areas, smooth, dull to shiny, faintly white maculate soredia: granular, common, in linear, marginal to submarginal soralia; isidia and pustulae: absent medulla: white with continuous algal layer lower surface: black with brown to mottled white naked zone peripherally, centrally rhizinate; rhizines: scattered, simple. Apothecia: rare, substipitate, up to 10 mm in diam.; margin: crenulate; disc: brown, often perforate ascospores: ellipsoid to broadly ellipsoid, 15-22 x 9.5-12 µm. Pycnidia laminal, immersed conidia: 10.5-11.5 µm long (Nash et al. ).

Photobiont: Data lacking.

Chemistry: Spot tests: upper cortex K+ yellow, C-, KC-, P-; medulla K-, C-, KC+ red or orange, P- Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with alectoronic acid (major) and a-collatolic acid (minor) .

Ecology: On hardwood trees (often among mosses) in relatively exposed positions.

Molecular data: Genbank

Distribution: Neotropics and extending well into temperate regions to the north and south, Macaronesia, throughout Europe and New Zealand.

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