Description: Thallus crustose, episubstratic, continuous to faintly rimose, white to pale yellowish white, sometimes delimited by a dark prothallus. Apothecia lecanorine, sessile to slightly constricted at base, 0.4-1(-1.2) mm across, with a dark reddish brown, often glossy, flat disc, and a smooth, persistent thalline margin. Thalline exciple corticate, the cortex 15-20 µm thick laterally, expanding to 40-85(-150) µm at base, with fine granules soluble in K (visible under polarized light), clearly separated from the medulla by a crystal-free zone; proper exciple colourless, with crystals soluble in K; epithecium red-brown to orange-brown, the pigment not dissolving in K, without crystals; hymenium colourless; paraphyses not or only slightly thickened at apex; hypothecium colourless, not inspersed with oil droplets. Asci 8-spored, clavate, very thin-walled, with a K/I+ blue, tall tholus penetrated by a faintly amyloid apical cushion, the wall K/I-, surrounded by a blue outer layer, Lecanora-type. Ascospores 1-celled, hyaline, ellipsoid, 11-17 x (5.5-)6-9(-9.5) µm. Photobiont chlorococcoid. Spot tests: thallus K+ yellow, C-, KC-, P- or P+ pale yellow. Chemistry: atranorin, plus a complex of unknown terpenoids.
Note: a mainly Mediterranean species found on smooth bark of isolated broad-leaved trees, much rarer in the North than in Mediterranean Italy. According to Malícek (in litt.) the synonymisation of L. sienae with L. horiza might prove to be incorrect: although the former is a very variable species, L. sienae would represent a real extreme in its variation range.