Buellia leptoclinoides (Nyl.) J. Steiner forme photophile.

   Crustose thallus up to 5 cm in diameter, rimose, areolate by irregularly sized areoles depending on their position on the thallus, 0.5-3 mm, cortex with large crystals clearly visible in polarized light, cream, pale yellowish-gray, grayish, with a very distinct black hypothallus (especially in case of contact with another thallus). Apothecia initially included in the thallus then slightly sessile, more or less numerous, 0.5-1 mm in diameter, disk flat at first then convex and sometimes deformed, brown, brown-black, black. Asci utriform, 8 spored, 60-70 x 18-25 µm. Brown spores, elliptical, sometimes slightly curved and kidney-shaped, sometimes very slightly warted (according to the literature!), walls thinned at the apices and thickened at the subapical level and the septum, 1-septate, 15-23 x 7-11 µm according to the literature, 17-21 x 10-11 µm according to our measurements. Frequent black pycnidia, bacilliform pycnidiospores, 4-5 x 1 µm. Photobiont: Clorococcales. C-, K+ yellow then slowly becoming brown, KC- or KC+/- yellow, P+ pale yellow. Medulla I-. Fairly rare species occurring on vertical or inclined siliceous rocks (in particular schists) in humid places receiving sea spray from the xeric supralittoral zone. There seem to be 2 taxa: a form well exposed to the sun: Buellia leptoclinoides forme photophile with a cream or slightly yellowish thallus and a form growing more in the shade: Buellia leptoclinoides forme sciaphile.

_ Very similar and easy to confuse with other species of saxicolous crustose lichens occupying the same habitat but in the field the latter have different colored chemical reactions. The most similar species (and which can grow nearby!) is Buellia subdisciformis, but this species grows on drier and less acidic rocks, has larger apothecia and is K+ red.

_ Also be careful on the Armorican sandstone with Buellia saxorum but this latter species is C+ pink-orange and has smaller spores.