Graticula

Graticula, formerly incorrectly named "Craticula", is a genus of Palaeozoic coralline alga. They form the framework of reef rocks in the Silurian of Gotland, from the Högklint, Slite and Halla groups.
The Graticulaceae closely resemble the Cretaceous Solenoporaceae, and are only really differentiated by their stratigraphic position.
"Graticula" mineralized with calcite.