I believe this one is of the Silurian period.
An image or drawing of one can be found on the Kentucky Geological Survey web page: http://www.uky.edu/KGS/fossils/stromatoporoids.htm
These creatures were reef builders and belong to the phylum porifera. They were once thought to be corals but scientists now classify them as sponges. Their distant relatives are alive today in the Caribbean and Mediterranean Seas and Pacific Ocean. There is a two page chapter on them in the Fossils of Ohio book on pages 64-65. It says they are hard to identify by species unless they are thin sectioned and studied at the micro level. Also only the top level was alive and had bumps.
The second picture shows the specimen upside down.
