Herteliana gagei (Sm.) J. R. Laudon

   Thallus crustose, superficial to 20-25 cm across, smooth or finely rimose-cracked in the center, in the shade typically blue-green, "duck egg", green-bluish, gray-glaucous, whitish to yellowish in places eroded, more creamy and bluish-grayish-white in places slightly exposed to direct light, hypothallus not always visible except in the case of mosaic, blackish. The thallus is often placed at the base of rocks and in contact with the ground, more rarely halfway up on rocks except at the edge of a stream or on rocks that are always wet. In the fertile form the apothecia are first immersed in the thallus and then become more or less sessile breaking the thallus, 0.5-0.7 mm in diameter, convex to tuberculous, often surrounded by a ring of the thallus more or less torn and with a purple halo, dark red-brown to blackish disc. NB This fertile form is rare and most often some apothecia do not reach maturity. Spores simple, fusiform with pointed ends, 18-22 x 8-10 µm. Pycnidia usually numerous in the form of small cones with a crater at the top, black. Photobiont: chlorococcoid. C-, K + yellow, KC + yellow, P-. Oceanic species on the base of humid siliceous rocks often in contact with the ground in the humid moors where this species seems to be an indicator of the Atlantic primary forest, rarer at the base of the old humid walls in the shade, common on the rocks in woods and oceanic forests. Cf. Lecidea phaeops which is P + yellow and whose numerous apothecia are black. This species is very variable and probably contains several taxon, we can however isolate certain forms (linked by intermediaries to the type):

_ More or less fertile form of very humid rocks in the shade and in the forest: Herteliana gagei forme fertile.

_ Sterile form on less humid rocks and less in the shade, most often on rocks and large stones of forest paths: Herteliana gagei forme saxicole des chemins.

_ Pycnidia form with only pycnidia: Herteliana gagei forme pycnidiée.

_ Sorediate form, Herteliana gagei forme sorédiée, probably endemic to Cape Lizard, English Cornwall and Pointe du Finistère, this form is related to the type by Herteliana gagei forme saxicole des rivages exposés. whose soredia are few and far between developed.