Tremolecia atrata (Ach.) Hertel

    Thallus crustose, to 3-4 cm diam., tightly appressed, areolate, rust-coloured, brown-orange, ochre-orange, protahllus irregular, sometimes present to conspicuous, blackish. Apothecia numerous, immersed within areoles, irregular, 0.3-0.5 mm diam., disc bright, plane then concave, black, exciple regular, conspicuous, prominent. Ascospores simple, ellipsoid, 10-16 x 6-9 µm. Photobiont: trebouxioid. Chemical spot tests negative. Continental trends, rare in our prospection area, on siliceous rocks or on heavy metals or iron enriched rocks (notably on tailings of lead mines), systematically in association with Rhizocarpon oederi that is very similar and rather difficult to distinguish on the field (and much more common). The latter can be distinguished by its gyrose apothecia and its more intense rust-colour, but in many cases it may require a microscopic analysis: Tremolecia atrata has simple ascospores and Rhizocarpon oederi muriform ascospores.*