Opegrapha saxigena Taylor

    Thallus crustose, thin, continuous, smooth, usually not cracked, brown-chocolate, brown-purple, sometimes with grey-brownish areas edged by a thin  brown-black prothallus, notably in case of mosaics, soralia absent. Apothecia lirelliform, scattered, simple and seldom branched,  0,5-1,2 x 0,2-0,4 mm, black. Ascospores ellipsoid, 3-septate, 16-20 x 4-5 µm. Thallus: C-, K-, KC-, Pd-. Rare, on sheltered and shaded siliceous rocks, also on old walls, rare on smooth tree bark: see Opegrapha saxigena forme corticole. May contribute to mosaics together with  Enterographa (Opegrapha) zonata and Opegrapha gyrocarpa of which the thalli are sorediate, but also with other crustose lichens thriving in this habitat.*