Thallus endolithic, white, grey or discoloured brown, with calcite deposits that often slightly raise the surface above that of the bedrock. Cortex poorly defined. Photobiont a green alga.
Anamorph: not known.
Teleomorph: ascomata perithecia, (170-) 260-520 (-660) µm diam., immersed in well-defined pits in the rock, occasionally with the apex projecting. Involucrellum absent. Peridium brown throughout (blackish in thick section, translucent brown in squash preparation), K± slightly darker brown or greenish-brown. Interascal tissue of periphyses and periphysoids, interascal hyphae absent. Hymenial gel I+ red (+ blue at very low concentrations of I), K/I+ blue. Asci narrowly ellipsoidal to clavate, fissitunicate with rostrate dehiscence, the wall thickened above and an ocular chamber usually present, 8-spored. Ascospores (28.5-)36-46 (-54) × (11-)13.5-16.5(-20) μm, length/width ratio (1.8-)2.5-3(-3.9), 3-(to 5-) septate, occasionally with one or two longitudinal septa, hyaline, smooth- and thin-walled, without a gelatinous sheath or perispore.
Assessed by Woods & Coppins (2012) as of Least Concern. The species appears to be common and widespread.
Characterized by the relatively large perithecia immersed in pits, absence of an involucrellum and mostly 3-septate spores.
Thelidium fontigenum differs in the smaller perithecia, smaller ascospores and frequently by the presence of an orange, K+ violet pigment. Polyblastia dermatodes is said to differ in the submuriform ascospores, but the number of longitudinal septa is variable even within a single specimen and it is difficult at present to make a clear distinction between the two species. There appears to be little or no correlation between the number of septa, the size or length/width ratio of the ascospores and the size of the perithecia; in addition, many of the collections which have submuriform spores appear to be in poor condition and some spores are clearly abnormal. This group of taxa requires careful revision.
Widespread throughout the UK where suitable rocks are present. BLS map: here.
On limestone and other calcareous rocks, common in graveyards as well as on natural outcrops.