Thelidium incavatum  

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.