Description: The large, black, thick-margined, convex discs belonging to this species may resemble those of Bacidia, Lecidea or Cresponea … but on the basic-bark of an ash or a maple? Look again. This time you see that the thallus is pale, grey or whitish, non-sorediate, non-isidiate. The discs are large (to 2mm diam.) with shiny raised margins. By a process of elimination you can identify this Megalaria in the field. For further confirmation cut a section through the disc and mount in K. Look at it under a microscope where the large, slightly waisted one-septate spores (20-30 x 8-15&

371;m) will be seen. Spot tests negative. Western Ireland.

Key Identification Features:

• Large, black discs with a thick shiny margin on basic or nutrient-rich bark; thallus neither isidiate or sorediate, pale
• Mostly in older woodland or large trees in sheltered river valleys.

Original text submitted by Vince J. Giavarini

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

iNaturalist: Megalaria grossa at iNaturalist World Species Observations database.