Lichenogr. Univ.: 363, 1810. Basionym: Lichen swartzii Ach. - K. Vetensk.-Akad. Nya Handl.: 185, 1794.
Synonyms: Lecanora glaucoma var. swartzii (Ach.) Nyl.; Lecanora pachycarpa Samp.; Lecanora rupicola var. leucogaea (Ach.) R. Sant.; Lecanora sordida var. swartzii (Ach.) Rabenh.; Lecanora subradiosa auct. non Nyl.; Zeora sordida var. swartzii (Ach.) Körb.
Distribution: N - Ven, TAA, Lomb, Piem (Isocrono & al. 2004, Isocrono & Piervittori 2008), VA (Piervittori & Isocrono 1999, Matteucci & al. 2015c), Emil, Lig (Watson 2014). C - Tosc (Tretiach & al. 2008), Sar (Roux & Triebel 1994). S - Cal (Puntillo 1996), Si (Grillo 1998, Grillo & Caniglia 2004).
Description: Thallus crustose, episubstratic, rimose to verrucose-areolate, white to yellowish grey, smooth, usually white-pruinose, often delimited by a white, more or less arachnoid prothallus, or sometimes almost placodioid. Areoles flat to strongly convex, thick, ecorticate. Apothecia lecanorine, constricted at base to almost stalked, 0.3-1.5 mm across with a brown or grey-brown, but heavily white- or bluish grey-pruinose, flat to convex disc, and a thin, even, smooth thalline margin without an inner parathecial ring. Amphithecium with an algal-free, strongly conglutinate eucortex and a more or less loose medullar plectenchyma in inner parts; parathecium hyaline, containing crystals soluble in K; epithecium brown to dark brown, with crystals, the pigment and the crystals dissolving in K; hymenium colourless; paraphyses slightly thickened at tips; hypothecium colourless. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-celled, hyaline, ellipsoid, 9-14 x 6-7.5 µm. Photobiont chlorococcoid. Spot tests: thallus K+ yellow, C+ yellow, KC+ orange, P+ pale yellow; apothecial disc (pruina) C+ bright yellow. Chemistry: atranorin (major), chloroatranorin (minor), eugenitol (minor), roccellic acid (major), and sordidone (major).
Note: an arctic-alpine lichen found on steeply inclined to underhanging surfaces of siliceous rocks, with optimum near and above treeline; most frequent in the Alps, but also occurring in the high Mediterranean mountains. The subsp. caulescens (J. Steiner) Leuckert & Poelt is known from the Austrian Alps.