Description: Thallus squamulose, heteromerous, pale greenish grey to brownish when dry, green when wet. Squamules 0.5-2(-3) mm broad, 0.15-0.25 mm thick, usually weakly to deeply lobed, convex, smooth, scattered to contiguous, notimbricate, fully adnate or rarely with a slightly up-turned margin. Lower surface black, with conspicuous black rhizines anchoring and linking the squamules, and colourless to dark rhizohyphae; rhizines moderately to richly branched, to several mm long, with a carbonaceous envelope and a colourless core. Upper cortex pseudoparenchymatous, 30-70 μm thick, overlain with an epinecral layer; lower cortex more or less paraplectenchymatous in upper part, brown-black; medulla white, subparaplectenchymatous, composed of rounded cells. Perithecia frequent, subglobose, fully immersed, up to 0.4 mm across, without involucrellum. Exciple brown-black to black throughout, 20-30 μm thick; paraphyses absent, substituted by periphyses; hymenial algae subglobose to ellipsoid-oblong; hymenium colourless, I+ brown-red. Asci (1-)2-spored, bitunicate, thick-walled, clavate to cylindrical-clavate, bi- and fissitunicate, thin-walled, the wall non-amyloid. Acospores muriform, at first hyaline, then pigmented, ellipsoid, (16-)28-60(-75) x 11-26 μm, the apical spore smaller. Pycnidia immersed, to 0.3 mm across. Conidia hyaline, bacilliform, 3-5 x <1 μm. Photobiont chlorococcoid (Stichococcus, present in both thallus and hymenium). Spot tests: Thallus and medulla K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: on calcareous soil, most often in fissures of calcareous rocks; the genus Endocarpon badly needs revision, E. pusillum in the sense of most European authors is heterogeneous, and perhaps could be subdivided into several species.