Hexagonaria is a genus of colonial rugose coral. Fossils are found in rock formations dating to the Devonian period, about 350 million years ago. Specimens of Hexagonaria can be found in most of the rock formations of the Traverse Group in Michigan. Fossils of this genus form Petoskey stones, the state stone of Michigan.
Hexagonaria is a common constituent of the coral reefs exposed in Devonian Fossil Gorge below the Coralville Lake spillway and in many exposures of the Coralville Formation in the vicinity of Coralville, Iowa.
Based on Erwin C. Strumm's Corals of the Transverse Group of Michigan Part 13, Hexagonaria, published in 1970.