Lecidea lapicida (Ach.) Ach.

    Thallus crustose, rather thin, rimose to areolate, whitish, pale greyish, grey-bluish, seldom with rusty spots on iron-rich rocks, sometimes delimited by a blackish prothallus. Apothecia emerging to sessile, at the edge or between areoles, sometimes in small clusters or in lines, 0.5-1.5 mm diam., disc flat to slightly convex, blackish, exciple angular, thin, concolourous or greenish, persistent. Ascospores broadly ellipsoid, simple, colourless, 10-15 x 5-7 µm. Photobiont: Trebouxia. Thallus: C-, K+ yellow (not always convincing) , Pd- or Pd+ faint yellow. Medulla: I+ blue-purple. Montane trends, on siliceous rocks. N.B. This species is subjected to various interpretations according to authors. Easily mistaken with a Porpidia and notably with Porpidia crustulata and Porpidia cinereoatra but the latter are medulla I-.*