Syringoporid
This little rock from Arkansas is a Pennsylvanian aged coral from Arkansas. This was a Syringoporid coral, a type of tabular coral. Syringoporid corals existed from the Ordovician until the end-Permian Mass Extinction. Tabulate corals, the larger order, continue to this day – they form large, flat sheets and grow smaller parts of the colony vertically from the large sheet. Different genuses of Syringoporid corals are classified based on properties such as the width of separate tabularium (The columns that hosted each live polyp), the spacing between the distinct coralites, and the thickness of the walls between them. Changes in these properties lead to slightly different coral morphologies in the fossil record.
-JBB
Image source: https://commons.wikimedia.org/wiki/File:Syringoporid.jpg