Among the most common fossils that I've collected from the eroded remains of the Louisville Limestone are Stromatoporoids, a type of calcareous sponge. They generally have two forms, the first is a mound shape:
Sometimes the many layers can look a bit like an onion when eroded or broken
The other predominant form is encrusting, often on coral or other Stromatoporoids.
Note the piece of Coenites coral that formed the nucleating point for the colony.
The two forms likely represent different genus and possibly families. Another explanation for the encrusting, layered variety could be that the colony frequently was partially or completely buried and subsequently recolonized the substrate. The Louisville limestone is Silurian in age (Homerian to Gorstian stage).