Description: This lichen has a mostly buried thallus with just a fine brown mottling visible; algae yellow or blue-green. The discs spherical (perithecia), flat at the centre, not sunken, up to half-immersed. Not in pits. Spores 20–30 x 8.5–12µm, one-septate, lower cell narrowed like an inverted and stretched acorn. It grows on damp limestone and flushed base-rich schistose rocks. It is known from north-west Burren, County Clare.

Key Identification Features:

• Tiny lime-loving lichen with yellow or blue-green algae, flattened perithecia and acorn-shaped, one-septate spores
• On flushed rock such as limestone or schist.

Original text submitted by Vince J. Giavarini

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

iNaturalist: Collemopsidium caesium at iNaturalist World Species Observations database.