Synonyms: Amphoridium crassum (Arnold) Servít; Bagliettoa crassa (Arnold) Cl. Roux; Thelidium crassum Arnold; Verrucaria calciseda DC. var. crassa (Arnold) Arnold; Verrucaria crassa A. Massal. 1852 non Eschw. 1833 nom.illeg.
Distribution: N - Ven (Lazzarin 2000b).
Description: Thallus crustose, endosubstratic, white to pale grey, very thick, rugulose-verrucose. Cortex (lithocortex) densely prosoplectenchymatous, of conglutinated hyphae intermingled with microcrystals; algal layer discontinuous, the photobiont cells in clusters; medulla prosoplectenchymatous, without oil cells (macrosphaeroids) in the lower part. Perithecia black, completely immersed in the rock and leaving deep pits when they fall off. Involucrellum absent; exciple subglobose to broadly pyriform, the wall black throughout, the pigment dark reddish brown, K+ dark grey-brown; hymenial gel hemiamyloid, I+ red (I+ blue at very low concentrations of I), K/I+ blue; hamathecium of periphyses in the ostiolar canal, and short periphysoids in the half upper part of the perithecial cavity; paraphyses dissolving early. Asci 8-spored, two-layered, fissitunicate, with a thin tholus reacting I-, Verrucaria-type. Ascospores 1-celled, hyaline, ellipsoid, 12-18 x 6-7 μm. Photobiont chlorococcoid. Spot tests: K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: a taxon of the B. calciseda-group with a thick, rugulose-verrucose thallus, and ellipsoid ascospores measuring 12–18 × 6–7 μm (according to Servít even larger); the distribution is still insufficiently known.