Circinaria calcarea (L.) A. Nordin, Savic & Tibell = Aspicilia calcarea (L.) Mudd.

    Thallus crustose, spread out and forming circular patches up to 20-25 cm in diam., smooth, finely cracked-areolate; areoles radially orientated especially at outer edge of thallus, whitish, dull white, white-grey to pale-grey, ochreish,  prothallus usually delimiting, pale-grey to dark grey, conspicuously zoned. Apothecia numerous, 1(-4) per areole, irregular in size and shape, 0,2-1 mm, more or less immersed, disc concave, black, slightly white-blue-pruinose when fresh, thalline margin thickened, chalkwhite, but only slightly raised and disappearing when old. Asci 4-spored, utriform (shaped like a leather bottle) with a markedly swollen apex, I+, 90-110 x 35-45 µm (according to our measures). Ascospores broadly ellipsoid to subglobose, 18-30 x 14-27 µm according to literature, 22-27 x 20-30 µm according to our measures. Photobiont : Chlorococcales. Thallus: C-, K-, P-. Not especially maritime, on hard limestones, and consequently rare in West Brittany. N.B. Close taxa more greyish seem to exist in Finistère on local rocks called kersantite. In hot and well exposed to sun this species is sometimes  parasitized by Heteroplacidium fusculum.