Description: Thallus of peltate, orbicular to irregular, scattered to crowded, cream-coloured to olivaceous brown squamules, starting the life-cycle on the thalli of Peltula-species. Squamules up to 15 mm in diam. and up to 0.9 mm thick, the upper surface smooth or often cracked, with an undulate margin. Lower surface reddish brown to brown-black, rugose, attached by a central holdfast or by centrally located, thick rhizohyphae. Upper cortex paraplectenchymatous, (45-)100-180 μm thick, often overlain by a thick epinecral layer; medulla white, prosoplectenchymatous, I-; lower cortex poorly developed, of periclinallly arranged, dark hyphae. Apothecia lecideine, black, epruinose, rounded or irregular in outline, up to 1.2(-1.8) mm across, marginal or laminal, sessile and slightly constricted at base, with a strongly convex disc and a thin, soon excluded proper margin. Proper exciple of radially arranged hyphae; epithecium blue-green to green, without crystals, N+ purple; hymenium colourless, 40-65 μm high, I+ blue turning reddish; paraphyses simple or sparingly branched and anastomosing, 2-3.5 μm thick at base, the apical cells to 6 μm wide; hypotecium pale brown. Asci 8-spored, clavate, surrounded by a thin gelatinous sheath reacting I+ blue, with an amyloid tolus of finely superimposed layers and a darker tubular structure, approaching the Psora-type. Ascospores 1-celled, hyaline, broadly ellipsoid to subglobose, (6-)8-12(-15) x (4-)5-7.5 μm. Pycnidia globose, hyaline, immersed in the squamules. Conidia straight, bacilliform, 5-8 x c. 1 μm. Photobiont chlorococcoid. Spot tests: cortex and medulla K-, C-, KC-, P-. Chemistry: low amounts of atranorin and an unidentified substance.
Note: on steeply inclined surfaces of siliceous rocks, growing on the thalli of Peltula-species in areas with frequent humid maritime winds; much less frequent than its hosts and mostly coastal in Italy.
Growth form: Squamulose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly sexual
On otherwise dry surfaces with short periods of water seepage after rain