Thallus to 2-3 cm diam., forming continuous smooth to granular patches, grey. Prothallus when present black. Soredia arising in ± circular, convex soralia, to 1 mm diam., farinose, pale yellow (to white in dried collections). Large crystal clusters occur in the thallus and are visible as translucent spots when the thallus is moistened.
Anamorph: no information available.
Teleomorph: ascomata apothecia, usually absent, to 0.8 mm diam., dispersed, immersed or sessile. Thalline exciple entire to delicately
crenulate, becoming excluded, including large crystal clusters. Discs pale green-, pink-brown or rarely almost black, flat. Epithecium pale yellow-brown interspersed with small granular crystals. Hymenium 35-60 μm tall. Interascal tissue of paraphyses, sparsely branched, apices not swollen or pigmented. Asci 45-50 × 10-15 μm. Ascospores (7-) 10-14 × (4-) 6-8 μm.
Chemistry: thallus C–, K–, soralia sometimes weakly K+ yellow, Pd–, UV± pale orange (± atranorin, ± chloratranorin, 2-O-methylsulphurellin and usnic acid).
Assessed by Woods & Coppins (2012) as of Least Concern.
A member of the Lecanora polytropa group. The discrete clusters of large oxalate crystals in the thallus of L. jamesii help distinguish it from other sterile crusts with a greyish thallus and green/yellowish soralia. The superficially similar L. farinaria, also occuring in carr woodland, has a different chemistry and much darker discs. Coccotrema citrinescens may resemble L. jamesii when it occurs on rocks, but the vivid K+ golden yellow reaction of the soralia of the former is diagnostic.
Locally abundant in SW and western England, very occasional in the Midlands and eastern England; widespread in Wales and western Scotland, scattered in Ireland. BLS map here.
On smooth bark, especially towards the base of deciduous trees, most frequently Salix in damp situations, rarely on wood or siliceous rocks.