Description: This endemic, shaded-limestone-loving Verrucaria is virtually invisible from above so close inspection of the substrate is required to locate it. The brown-grey, cracked vaguely superficial thallus has discs (perithecia = 0.12-0.3mm diam.) fully visible to semi-immersed but not covered by a layer of thallus. The discs are, however, covered by a shield of tissue — the involucrellum. The average spore size is in the range 9 x 6 µm. In Ireland the species is known only from the Aran Islands.

Key Identification Features:

• Thallus semi-immersed and easily missed or superficial brownish-grey
• Discs with an involucrellum but no thalline covering; distinguished from V. bulgarica by the larger perithecia.

Original text submitted by Vince J. Giavarini

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

iNaturalist: Verrucaria knowlesiae at iNaturalist World Species Observations database.