Lasallia pustulata (L.) Mérat

    Thallus monophyllous, resembling to a small burned crepe with more or less grilled parts, 8-10 cm diam., upper surface covered with numerous, crowded, conspicuous pustules, like toad skin, satiny or minutely cracked, whitish to pale ochre-greenish towards the center when wet, brown-milk-coffee, dark brown when dry, margin darker, covered with brown-black coralloid isidia, lower surface reticulate, roughened, brown-greyish. Very seldom fertile. Apothecia 1-3 mm diam., disc flat smooth, black. Ascospores muriform, black, 28-70 x 18-34 µm. Photobiont: Trebouxia. Medulla : C+ red, K-, KC+ red, Pd-. On nutrient-rich substrates, not especially maritime, common on siliceous rocks, standing stones, birds' perching rocks and boulders, sometimes forming spectacular swards of large numbers of thalli in nutrient-enriched seepage tracks on rock ridges and steep faces. Not to be confused with the much less common Umbilicaria polyphylla of which the lobes are not pustulate and margins without isidia. See also Lasallia brigantium, lacking coralloid isidia and most often fertile and restricted to Corsica and Sardinia.