Description: Encrusting, up to 5 cms in diameter and 1 mm thick. May have orbital ridges. Growing irregularly flat and smooth without upright branches. Shows "watermarks" - wave-like marks on the surface.

Habitat: Widespread and common. Epilithic on rock or stones, in the mid to lower littoral to the sublittoral at 35 m or more.

Distribution: Throughout the British Isles. Europe: France, Norway, Faroes, the baltic and Iceland. Greenland. Atlantic coast of North America from Arctic Canada to Connecticut.

Similar Species: Phymatolithon lenormandii (Areschoug) Adey Phymatolithon lamii (Lemoine) Chamberlain.

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

iNaturalist: Phymatolithon laevigatum at iNaturalist World Species Observations database.

WoRMS: Phymatolithon laevigatum at World Register of Marine Species.