I am trying to figure out if these are coral pieces or bryozoans.
REVISED (06/2008): It appears these are pieces of a branching coral described by Carl Ludwig Rominger (aka Charles Louis Rominger) as Dendropora alternans in 1876 (p.64, Plate 24, Figure 1) in his book Geological Survey of Michigan Lower Peninsula 1873-1876 Volume III Part II Palaeontology - Corals.
It is known today as Thamnoptychia alternans and described like that by E.C. Stumm in 1950.
Still some of the pieces I have are grooved with an almost fingerprint like pattern so maybe that is weathering but I have not found an image yet that looks like it. After looking at CORALS OF THE DEVONIAN TRAVERSE GROUP OF MICHIGAN PART V, TRACHYPORA
BY ERWIN C. STUMM and ALLEN S. HUNT paper and reading the description of this coral it seems that outer texture pattern might not matter in classification. Of course, I might be interpreting the academic speak wrong in this paper as well.
I continue to revise this entry but now after looking at Kentucky fossil corals, a monograph of the fossil corals of the Silurian and Devonian rocks of Kentucky, Pt. 2: Kentucky Geological Survey, xiii p., 139 pls. It appears to that surface pattern is on Plate 65, Figure 10 called Dendropora osculata or today called Trachypora osculata (Stumm). See this http://www.uky.edu/OtherOrgs/KPS/fossilcorals/images/davispl065.jpg