Halysites, commonly called "chain coral," is a coral that lived from the Ordovician to the Silurian Period. It looked like a bunch of straws or tubes linked together. From above, Halysites looks like a mass of chains or a brain.
From the side it looks like a fence.
Each individual corallite, or cell, is 2 to 6 mm in diameter. In each individual tube there was a tiny sea anemone-like animal called a polyp, similar to what you find in modern corals.
Corals today prefer warm, shallow, clear seas, so tabulate corals like Halysites and their relatives probably did too. So when sea levels rose in the Devonian, they became rare and then died out two periods later, in the Permian mass extinction.