Description: The thallus of this species can be grey or white, variably thin and superficial or occasionally immersed. The large discs (perithecia = 0.3–0.7mm diam.) form dots on basic siliceous rock surfaces and limestone. The discs possess an outer sheath (involucrellum) but are not covered over by a layer of thallus. The one-septate spores have an average size range of 21–28 x 10–13µm. There are only a handful of Irish records.

Key Identification Features:

• Thallus white or grey, visible or immersed; perithecia with an involucrellum but lacking an additional layer of thallus covering the top
• Spores one-septate, medium sized; dry, basic siliceous rock and limestone.

Original text submitted by Vince J. Giavarini

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

iNaturalist: Thelidium pyrenophorum at iNaturalist World Species Observations database.