Rhizocarpon reductum Th. Fr.
Rhizocarpon obscuratum auct. pp.
Thallus crustose, rather thin, small, to 1-2 cm across but able to cover large surfaces by conspecific colonies confluence, cracked, areolate, areoles 0.2-0.4 mm across, flat to slightly convex, grey, grey-brownish, prothallus black, more or less developed at the margin, larger when shaded. Apothecia innate to slightly sessile, 0.4-0.8 mm diam., disc flat to a little convex at first then markedly convex when mature, rough, black, exciple persistent, thick, black. Ascopsores ellipsoid, muriform, 20-35 x 10-15 µm. Photobiont: Chlorococcoid. C-, K+ yellow, Pd+ orange ; seldom K-, P- or K+ red, Pd+ orange. Not especially maritime, but rather common on large pebbles and siliceous rocks. Exceptionally variable and several ecotypes have been described and among them some more specifically maritime. N.B. Easy to confused with Fuscidea cyathoides (Pd+ red) and with species belonging to the Buellia aethalea group (K+ yellow > red) and sometimes found mixed together, requiring an observation of mature ascospores to be distinguished. Additionally, not to be confused with Rhizocarpon richardii that is darker with violet tinge and with Rhizocarpon petraeum on calcareous rocks, of which the thallus is thinner, less dark and more whitish, not cracked up to the margin and with a less developed prothallus.*