Cystiphyllum horn coral from the Silurian Period. It was found in central Jefferson County, Kentucky in the Louisville Limestone. One key to identifying this specimen are little circles that looks like bubbles were once there.
Identification from Erwin Stumm's Silurian and Devonian Corals of the Falls of the Ohio pp. 51-52 (description) and plate 6 pp. 96-97.