Description: Look for this crustose species on damp, basic rocks such as limestone, calcareous sandstone, mudstone and basalt alongside streams and rivers or where there is seepage over rocks. The thallus is immersed so all there is to see is a grey or brown staining of the rock surface sprinkled with dot-like discs. As with all Thelidium lichens, the discs are spherical (perithecia); in T. papulare these are large (up to 0.8mm diam.) and project well above the surface with a thick, black, outer sheath (involucrellum), forming a slight depression at the apex where the ostiole (spore exit point) is situated. Spores are three-five septate. North and central Ireland.

Key Identification Features:

• Relatively large perithecia raised well above the surface with thick black capping or involucrellum
• Large, mostly three-septate spores; T. pyrenophorum is close but has smaller, one-septate spores.

Original text submitted by Vince J. Giavarini

Distribution Map from NBN: Thelidium papulare f. papulare at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Thelidium papulare f. papulare at iNaturalist World Species Observations database.