Thallus crustose, dull green, grey or fawn-brown, finely scurfy granular-sorediate. Soredia often in rather discrete soralia. Granules (goniocysts) 20-50 μm diam. Photobiont cells 5-13 μm diam.
Anamorph: conidiomata pycnidia, immersed, white or pink-brown around the ostiole. Conidia 29-35 × 1 μm, curved.
Teleomorph: ascomata apothecia, 0.2-0.6 (-1) mm diam., flat or later irregularly convex, reddish brown to brown-black, the margin often darker than disc or becoming excluded. Thalline exciple absent. True exciple hyaline except for a red-brown, K+ purple, outer edge, with cellular lumina (to 7 μm diam.) towards the outer edge and in the lower part. Hymenium 45-55 μm high, hyaline or pale purple-brown above, K+ intensifying purple. Hypothecium hyaline. Interascal tissue of unbranched paraphyses 1-2 μm diam., the apices often swollen to ca 5 μm diam. Asci clavate to cylindrical-clavate, with a K/I+ dark blue apical dome and a pale, ± conical apical cushion, 8-spored. Ascospores 24-48(-63) × 1.5-2(-2.5) μm, acicular, with 3 to 7 narrow septa, hyaline, thin-walled, smooth, without a perispore or gelatinous sheath or appendages.
Chemistry: thallus C–, K–, KC–, Pd–.
Assessed by Woods & Coppins (2012) as of Least Concern, but considered to be Nationally Scarce.
Characterized especially by the often rather discrete soralia. Similar to Bacidia adastra, but with shorter conidia (25-35 μm), a thinner thallus of farinose soredia and a tendency to form discrete soralia, an exciple that becomes excluded and a somewhat different ecology. Furthermore, it contains a pinkish to red-brown, K+ purple pigment in the exciple and completely lacks a green pigment.
The species has a scattered distribution extending from Cornwall to Moray but with a broadly eastern pattern, with a concentration of records in East Anglia. BLS map: here.
On sheltered calcareous rocks and stonework, asbestos-cement and mosses on crumbling mortar, more rarely on bark (e.g. Alnus, Salix, Sambucus, Ulmus), bare wood and rabbit droppings