Caloplaca littorea Tav.
Thallus crustose in small to extensive patches and more or less cracked and rounded, bright orange-yellow so well exposed to the sun, golden yellow to yellow if sheltered from the sun, rarely bordered by a pale yellow hypothallus and even more rarely embryos lobed, golden-yellow; more or less covered with small simple finger-like or coralloid isidia sometimes giving it a velvety appearance if there are many, yellow, yellow-orange, orange depending on the exposure. Apothecia rare, often scattered, to 1 mm diam., disc rounded, flat, orange; proper margin orange, often becoming excluded when old. Ascospores ellipsoid, polarilocular, 11-15 x 5-7 µm, septum 1-4 µm wide, to about 1/4 of the length of the ascospore. Photobiont : trebouxioide. All parts K + purple. Uncommon and localized species coming from siliceous rocks little exposed but sunny, especially in horizontal cracks or on vertical faces.
N.B. On the same site it is possible to find specimen entirely covered with isidia, hiding the thallus, and other specimen with very few isidia leaving a thallus reduced to a mere yellow-orange rough crust. Very difficult to distinguish from Caloplaca britannica which has minute radiating lobes, globular isidia, and is paler yellow and growing within the Verrucaria (Hydropunctaria) maura zone and from Caloplaca citrina ss. auct. brit. which has granular soredia, more yellow-lemon on less acid and more humid rocks.