Distribution: N - Frl. C - Abr (TSB 30590), Mol (Caporale & al. 2008).
Description: Thallus squamulose, homoiomerous, slightly gelatinous when wet. Squamules erect, peltate, brown-black to black, rather shiny, dispersed to crowded, to 3 mm thick, 1-8 mm in diam., often richly and reticulately fissured, not pruinose and not isidiate, attached by a central holdfast of rhizohyphae. Apothecia 5-9 per squamule,immersed in the thallus, first punctiform then expanded and clearly lecanorine, 0.2-1 mm across, with a dark reddish brown, first urceolate then flat disc, and a thick, smooth thalline margin. Epithecium orange-brown, 5-10 µm thick; hymenium colourless, 60-100 µm thick, the hymenial gel I+ blue; paraphyses to 5 µm thick, moniliform in upper part; hypothecium yellowish, 30-90 µm thick in central part. Asci 8-spored, subcylindrical, thin-walled, with an amyloid apical cap. Ascospores 1-celled, hyaline broadly globose or subglobose, 9-14 x 7-10 µm. Pycnidia globose, immersed. Conidia ellipsoid or bacilliforn, 2-4 x c. 1.5 µm. Photobiont cyanobacterial, chroococcoid, of a few cells 7-12 µm diam. surrounded by a gelatinous sheath which is yellowish brown to purple near the thallus surface. Spot tests: all negative. Chemistry: no lichen substances. .
Note: on steeply inclined, sunny surfaces of calcareous rocks with periodical water seepage after rain. Italian material, congruent with two specimens from STU identified by A. Henssen (Marburg) was referred by Nimis & Tretiach (1999) to A. moedlingense Zahlbr., a variously interpreted taxon (see e.g. Henssen & Jørgensen 1990, Moreno & Egea 1992, Hafellner & Türk 2001), which lacks the characteristic spherical outgrowths of A. tumidulum (see also Zahlbruckner 1898). The suberect, deeply sulcate squamules with reticulate surface are diagnostic.