Thelocarpon periphysatum sp. nov. is described from marl quarries in the Netherlands. The species is characterized by perithecioid ascomata that have a green-yellowish ring around the ostiole, abundant periphyses and periphysoids that are up to 120 μm long, the absence of paraphyses and the wide, oblong and often somewhat asymmetrical ascospores. The perithecia are immersed in black cyanobacterial crusts on calcareous rocks. A worldwide key is provided to the 30 species of Thelocarpon that are currently accepted.
