Here is a Cystiphyllum niagarense? horn coral from the Middle Silurian Period found in central Jefferson County, Kentucky. Corals like this are found in the Louisville LimestoneCyst is from the Greek kystis meaning sac, pouch or bladder. The horn coral appears to have been covered with small bubbles or sacs. You can see the small places showing the little areas on top of the horn coral.
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.