Caloplaca arenaria (Pers.) Müll. Arg.

     Thallus crustose, hardly visible, immersed in substratum -often crustose lichen itself- pale to dark grey-brown. Apothecia numerous, 0,3-1 mm diam., clustured or scattered, rounded to flexuose, disc flat, dark orange, bright red, proper margin prominent, ± elevated, persistent, sometimes minutely crenulate, concolourous. Asci 8-spored. Ascospores ellipsoid, polarilocular, septum 1.5-3 µm wide c. 1/5 -1/6 of the length of the ascospore, 12-17 x 5-8 µm. Photobiont : trebouxioide. Thallus : K-, Apothecia: K+ purple. Not especially maritime but found on seashore siliceous rocks, frequently overgrowing other crustose lichens, often associate to Rhizocarpon notably of the  Rhizocarpon geographicum group. Ressembles to Caloplaca ceracea but differs by its small apothecia and its thallus hardly visible, and to Caloplaca crenularia of which the apothecia are larger and dark red, almost ferrugenous, the thallus thicker and ascospores with a long equatorial thickening. N.B. A species very similar to Caloplaca subpallida which is distinguished (sometimes with difficulty) by its more orange colors and its larger spores, but this last species is not always separated by all the authors, in particular the British.