Description: Thallus crustose, endosubstratic, ecorticate, continuous, white to brownish white, sometimes with a greenish tinge, delimited by a concave, concolorous prothalline line. Apothecia 0.15-0.25(-0.3) mm across, urceolate, immersed to half-immersed, at first covered by a thalline layer and appearing perithecioid, later erumpent and with a concave to flat, brownish, 0.1-0.15 mm wide disc and a usually smooth, 0.05-0.15 mm thick margin consisting in a very thin proper margin and an up to 0.1 mm thick thalline layer. Proper exciple 35-40 µm wide laterally, much thinner in lower part, paraplectenchymatous, of thick-walled, isodiametrical or elongated cells; hymenium colourless, (75-)90-160 µm high, I+ blue; paraphyses thread-like, 1.5-2 µm thick at base, not inspersed with oil droplets, the apical cells 2.5-3 µm wide; hypothecium poorly developed. Asci 8-spored, cylindrical-clavate, thin-walled, without a tholus, 57-75 x 9-14 µm. Ascospores (2-)3(-5)-septate, narrowly fusiform, straight or slightly curved, (13-)14-21(-24) x (4-)4.5-6(-7) µm, not halonate. Photobiont trentepohlioid. Spot tests: K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: a rare species described from southern France, resembling G. hypoleuca, but with smaller apothecia, shorter asci and mainly 3-septate, shorter ascospores, growing on steeply inclined, shaded surfaces of calcareous rocks in the Mediterranean belt (for further details see Roux & al. 2008). To be looked for in Mediterranean Italy.
