Tylothallia biformigera (Leight.) P. James & H. Kilias
Thallus highly variable, +/- thick, conspicuously rimose and cracked, often markedly verrucose, warts looking like aborted apothecia, clustered in globose heaps or cushions, grey-greenish, greenish when moist, prothallus black, +/- visible between wart clusters, but distinct in case of contact with another crustose lichen. Apothecia isolated or clustered, and in that case distorted, flat at first then tuberculate, 0.5-1.5 mm diam., disc flat to rough to verrucose, olive-brown, black > black-bluish, exciple wavy, thick, soon excluded, paler than the disc and often powdery. Ascospores elongate, ellipsoid with rounded apices, 1-septate, 10-15 x 3-5 µm. C-, K+/- yellow to orange, Pd+/- orange. Photobiont: Chlorococcoid. Rare, mainly on dry faces of vertical siliceous rocks of the foreshore in the mesic supralittoral zone and less frequently on old walls. N.B. Not to be confused with Cliostomum tenerum that, according to literature, may have a similar aspect, but is less greenish, Pd+ red with fusiform ascospores and grows in sheltered and dry rocky places. Recollect that the very altered specimen of the more nitrophile Lecidella scabra has a more green-bluish thallus due to numerous soredia and apothecia not tuberculate. Not to be confused with Lecidella asema , C+ orange.