Diploicia canescens (Dicks.) Massal.

    Thallus forming rosettes, 3-8 cm diam., across with clearly defined contiguous lobes radiating from the center, up to 1 mm wide, pleated, convex, pruinose, whitish, pale grey, grey-glaucous, granular or farinose soralia dispersed in the center, sometimes in patches or mats, dull grey-yellow, occasionally greenish due to algae. Seldom fertile in our prospection area, but fertile thalli may commonly be found near the sea shore, apothecia 0.3-1 mm diam., disc black, thin black margin. Ascospores broadly ellipsoid, brown, polarilocular,  1-septate, 10-15 x 5-8 µm. Photobiont : chlorococcoide. Usually: C-, K+ yellow → brown-yellow, KC+ yellow → brown-yellow, P- but there are many chemotypes in particular a P+ yellow chemotype, see: Diploicia canescens forme P+ jaune. Very common on basic and very nutrient-enriched substrata, tolerant to pollution, not especially maritime but rather abundant near the sea shore particularly on dikes, walls, ageing harbour facilities, bark of trees in cities, etc. see: Diploicia canescens forme saxicole, rarer on wood: Diploicia canescens forme lignicole, or on plastic substrates: Diploicia canescens forme sur substrats artificiels.

N.B. In this highly polymorphic species, the frequence of well developed thalli is rather low compared to those of reduced or incomplete thalli found on cement walls in wasteland and requiring much more attention to be identified. There are also many chemotypes.

N.B. In the Mediterranean region or in Macaronesia, but also in Brittany in hot areas, a very similar taxon may be found: Diploicia subcanescens., thermophilic, on coastal siliceous rocks with abortive soralia restricted to greyish warts.