Description: Thallus foliose to subfruticose, heteromerous, dorsiventral, up to 7 cm wide (usually much less), the lobes subcylindrical, 1-3 mm wide, at first adpressed, then soon ascending, up to 3 mm thick, hollow, mostly convex and smooth, dichotomously branched, grey above, black beneath, the tips bearing capitate soralia with farinose soredia. Lower surface black, wrinkled, erhizinate, attached only in the central or basal parts of thallus, otherwise free. Upper cortex of tightly packed, more or less anticlinally oriented hyphae, the cell walls with Cetraria-type lichenan; medulla soon becoming hollow, lining the cavity inside the lobes; lower cortex dark, paraplectenchymatous. Apothecia very rare (not observed in the study area), shortly stalked, lecanorine. Epithecium brown; hymenium and hypothecium colourless. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-celled, hyaline, subglobose or broadly ellipsoid, 6.5-8.5 x 4.5-5.5 μm. Photobiont chlorococcoid. Spot tests: upper cortex K+ yellow; medulla K-, C-, KC+ fleeting pink, P-, UV+ pale violet-blue. Chemistry: upper cortex with atranorin; medulla with physodic and 3-hydroxylphysodic acids.
Note: a mainly temperate, holarctic species of acid bark, often occurring on twigs; certainly rarer than H. physodes, and bound to more natural and humid situations.
Growth form: Foliose, narrow lobed
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)