Description: Thallus crustose, episubstratic, (35-)50-150 μm thick, subgelatinous when wet, mostly verrucose, continuous, dull green to dark grey-green or dark green, rarely with sparse, small black dots, delimited by a whitish prothallus or, from conspecific thalli, by black lines. Cortex paraplectenchymatous, poorly differentiated, the cell walls dull green; photobiont cells arranged in vertical rows; medulla paraplectenchymatous, the uppr part with small, upwardly projecting areas of dark cells appearing as more or less discrete dots in section, the lower part with black patches which often merge to form a black basal layer. Perithecia completely immersed in the thalline warts, covered with a thalline layer, usually visible as black dots, more rarely slightly projecting, the apex appearing as a whitish ring or as a black, 0.2-0.3 m wide, often roughened disc. Involucrellum well-developed in the upper half of exciple, tightly adpressed to exciple, sometimes reaching the black basal layer; exciple 275-290 μm across, colourless or partly brown; hamathecium of periphyses and periphysoids, interascal filaments absent, the periphyses 20-30 x 1.5-3 μm; hymenial gel hemiamyloid, I+ red (I+ blue at very low concentrations of I), K/I+ blue. Asci 8-spored, clavate, I-, fissitunicate, the wall thickened above, with an ocular chamber, dehiscent by extrusion of an endotunica to form a delicate rostrum, Verrucaria-type. Ascospores 1-celled, hyaline, oblong-ellipsoid to ellipsoid, (10-)14-18(-20) x (5.2-)7.5-9(-10) μm, without a perispore. Pycnidia black, immersed. Conidia straight or slightly curved, 3.5-5 x 1-1.2 μm. Photobiont chlorococcoid. Spot tests: thallus and medulla K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: on siliceous rocks in the amphibious zone of streams, often near rapids and waterfalls, but also on lake shores; widespread in Europe but only locally abundant; with a few records from the Alps outside Italian territory, but perhaps overlooked or misidentified as H. rheitrophila.