Description: Thallus leprose, cottony, cream-coloured, whitish or greenish white, sharply delimited, forming orbicular rosettes at least when young, sometimes almost sublobate, the upper surface often radially folded, consisting of a thick, soft mass of powdery, convex, soredia-like granules forming a compact, membranous crust which is easily detached from the substrate. Granules up to 60-200(-400) µm in diam., with projecting hyphae. Medulla white, usually developed, the hyphae 1.5-5 µm thick, covered by numerous colourless granular crystals; lower surface without a layer of dark grey or brown hyphae. Photobiont chlorococcoid, the cells spherical, c. 13 µm in diam. Spot tests: K+ yellow, C-, KC- or KC+ yellow, P+ reddish orange (but medulla P-), UV-. Chemistry: atranorin; different chemotypes, with or without protocetraric, fumarprotocetraric, roccellic, stictic, psoromic acids.
Note: on rocks, mostly on limestone but also on calciferous siliceous rocks, on mosses, more rarely on dust-impregnated subneutral bark, on steeply inclined or underhanging faces sheltered from rain, with a wide altitudinal range; certainly occurring throughout northern Italy. The sample from Campania, chemically analyzed by H. Kümmerling, is from Paestum (Seaward, in litt.).
Growth form: Leprose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)