Description: Small encrusting disc up to 5 mm in diameter with thin margins and a smooth surface of imbricating layers (lobes overgrowing). Conceptacles forming domed and raised chambers.

Habitat: Fairly common but poorly recorded. Epiphytic on other algae or epilithic on stones. Littoral and sublittoral to about 8 m depth.

Distribution: Throughout the British Isles especially on south-western coasts of England, Wales and Ireland. Europe: Mediterranean. Further afield: Canary Isles and Australia. Probably cosmopolitan.

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

iNaturalist: Hydrolithon boreale at iNaturalist World Species Observations database.

WoRMS: Hydrolithon boreale at World Register of Marine Species. Accepted name: Hydrolithon boreale (Foslie) Y.M.Chamberlain, 1994. AphiaID: 145116.

Classification: Biota; Plantae; Biliphyta; Rhodophyta; Eurhodophytina; Florideophyceae; Corallinophycidae; Corallinales; Hydrolithaceae; Hydrolithoideae; Hydrolithon