Thallus partly superficial (sometimes evanescent), indistinct, crustose, more evident around apothecia, pale grey to greenish grey, often with distinct bluish pigment. Apothecia occurring singly, or clustered in groups, sessile, or constricted at the base to almost raised, flat when mature or flexuose, 0.4–1.0 mm diam.; disc plane to convex, smooth, shades of yellowish, brown to blackish, or +/- plumbeous, slightly to heavily pruinose, rarely epruinose; margin level with the disc, or weakly prominent, smooth or rough, entire, even, uniform, pruinose, paler than thallus and paler than disc, white or grey or indistinctly delimited.  Amphithecium 160–170 μm thick, corticate, with algae densely filling the area below the cortex; cortex usually distinctly delimited, slightly thicker at the base than at the sides, 15–50 μm thick laterally and 45–60 μm thick at the base, composed of adglutinated hyphae, obscured by granules occasionally interfering into the area below the cortex (pol+, insoluble in K, soluble in N), thin and bluish-green at the very top of margin – due to coloured tips of marginal hyphae and bluish-green intracellular pigment (this part becoming purple in N); parathecium usually distinct, prosoplectenchymatous, 10–15 μm wide; epithecium yellowish or shades of brown or olive, sometimes partly bluish green, granular (pol+ or pol+/-), granules usually sparse, between paraphyses tips, coarse (soluble in K and insoluble in N), epipsamma usually present (insoluble in K, soluble in N); hymenium hyaline, 45–65 μm high; subhymenium indistinct; hypothecium hyaline or yellow (more intense in K), 50–100 μm high, composed of adglutinated hyphae, distinct from parathecium. Paraphyses simple or dichotomously branched at tips, slender or thickened, not expanded, or slightly expanded apically, often pigmented at top, free in K. Asci clavate, 8-spored; ascospores hyaline, simple, broadly ellipsoid (broadly ellipsoid to ellipsoid), 9–[10.6]–12 × 6–[6.3]–7.5 μm, Q = 1.4–[1.7]–2.0.  Pycnidia not seen.

CHEMISTRY. Apothecial margin K– or K +/- yellow, C– or C+ yellow, KC+ yellow, PD–; disc K– or K +/- yellow, C–, KC +/- yellow, PD–; apothecia UV– or some apothecia UV+ yellowish. Lichen products: vinetorin (5-chloro-3-O-methylnorlichexanthone); sometimes only traces detected by TLC.

HABITAT. Directly on noncalcareous or weakly calcareous rock, often overgrowing or parasitic on other lichens, e.g., Aspicilia spp., Lecanora spp., Verrucaria spp.

DISTRIBUTION. Europe, Asia and North America. In North America the species is scattered in boreal and temperate regions with the main distribution temperate, but it has also several localities in the Arctic region. 

DISCUSSION. Lecanora invadens is a morphologically variable species. For example, thallus abundance, apothecial disc colour and the presence/absence of pruina vary. Its epithecial granule type and chemistry are consistent, however.  The species is closely related to L. semipallida but differs in having a dark brown to blackish, epruinose or heavily pruinose apothecial disc and blue-green epithecium, and often a more distinct thallus. Its close relation to L. semipallida is indicated by the presence of K-soluble granules in the epithecium (sometimes sparse!) and the presence of vinetorin. The taxonomic status of the species should be reconsidered after more material from Europe is studied, to determine whether it merits species level.

NOMENCLATURAL NOTES. Lecanora dispersa var. coniotropa is included here as a synonymfor L. invadens H. Magn. although it was consideredconspecific with L. perpruinosa by Froberg(1989). The author referred to the collection by Magnusson originating from Torne Lappmark(northern Sweden) and to the collection by Poelt, Steiner and VÄ›zda from Switzerland (distributed in VÄ›zda, Lich. Selecti Exsicc. 639). Since I was not able to find the original collection by Fries, I referred to the material studied by Froberg (1989) here. For further discussion, see also under L. perpruinosa.

REMARKS: The affinities of the material of L. dispersa f. coniotropa (coniopta?) reported from France need separate study since, as characterized by Roux (1976: 23), it may represent a different taxon.