Acarospora subrufula (Nyl.) H. Olivier

   Thallus areolate, most often hardly visible, masked by apothecia invading areoles; areoles 0.3-2 mm wide, markedly contiguous, often in lines following minor fissures in the rock surface, edges rounded or irregular by compression, swollen, convex, pale brown-red to brown-chocolate. Apothecia numerous, round to irregular due to compression, aggregated in rock fissures, mostly 1(2-4) per areole; with areole they form a complex slightly prominent and globose, disc plane or sunken, chocolate to red-brown, giving the impression in the areole to be bordered with a thick prominent margin, lip-shaped, paler than disc and fading when drying, pale brown-chocolate, milk-chocolate, dark chocolate sometimes dotted or crenulate with minute whitish dots. Asci 100-spored. Spores narrowly ellipsoid, 4-6 x 1-2,5 µm. Photobiont: Chlorococcales. Chemical spot tests: C-, K+ yellow then dark wine-red, blood red, (coloration sometimes very bright when fresh on old apothecia very exposed to the sun, but also sometimes difficult to highlight on young apothecia less exposed to the sun), KC+ red-wine, red blood, bright on fresh samples, Pd+ yellow-orange (these reactions are only clearly visible on apothecia exciple for herbarium samples). Thermophilic and slightly nitrophilic species growing on granite rocks or siliceous rocks with very rough large crystals, well exposed to South on coastal fringe, almost at its North limit at Finistère head.

N.B. Restricted habitat, rare, difficult to found since hardly visible in minor rock fractures but characteristic due to its peculiar aspect, its habitat and its chemical reactions.

N.B. Red brown-chocolate when well exposed, turning blackish and globose with distorted and hardly visible apothecia in humid zones, when shaded. In that case, may resemble to Trimmatothelopsis versipellis that grows in the same habitat.