Lepraria membranacea (Dicks.) Vain.

    Thallus membranous, 5-15 cm across, most often delimited, on bare rocks, often forming  rough rosettes, margin lobate, irregular and carved, extreme lobe margins typically erect, 1-5 mm, on mossy rocks lobes may even be more distinct and cover the mosses, upper surface leprose or granulose, granules to 0.05-0.1 mm diam., up to 0.3 mm in the most ancient sections, pale grey-greenish when moist, but typically cream, grey-yellowish to yellow-ochre when dry, lower surface dark and tomentose-felted, medulla white. Apothecia unknown. Photobiont: green algae. C+ yellow, K+/- yellow, Pd+ red-orange. Not especially maritime, on shaded vertical walls of siliceous rocks or on stone walls, sometimes spreading and potentially covering the local mosses, seldom on old tree trunks. N.B. The typical specimen are easily recognized due to their dirty yellow thalli.*