Lecanora intumescens

Thallus continuous, smooth or minutely cracked, only becoming areolate in old central parts of the thallus, often poorly delimited, pale grey; prothallus black or absent. Apothecia (0.8–) 1–2.5 (–3) mm diam., sessile, slightly constricted at the base, dispersed or rarely aggregated when they may be angular by compression; thalline margin well-developed, appearing swollen, persistent, entire to flexuose, raised, white-grey, often pruinose, without a differentiated cortex, with small crystals; disc variably coloured, from pale to mid brown, also orange-, red- or dark brown, pruinose when young but becoming convex and non-pruinose with age; epithecium interspersed with coarse yellow granules dissolving in K; hymenium (70–) 80–110 µm tall; paraphyses 1.5–2.5 µm diam., unbranched or sparsely branched and anastomosed, apices to 3.5 µm diam., slightly swollen, colourless to yellow-brown. Ascospores 11.5–18 × 5–8 µm. Thallus C–, K+ yellow; exciple Pd+ deep yellow, UV+ pale orange to yellow (atranorin, chloratranorin, ± psoromic and 2′-O-demethylpsoromic acids, ± lichexanthone, zeorin).

A distinctive species of the L. subfusca group in its broad sense, characterised by the regular swollen white decorticate margins of the apothecia and (usually) the presence of psoromic acid (Pd+ deep yellow). A related species has been known for sometime from the Scottish Highlands and was described Arup et al (2023) as Lecanora caledonica. It is similar to L. intumescens, and shares the white decorticate margins, but the disks are medium brown to paler beige and never pruinose and the thalline margin is Pd–.