I think these are imprints of rostroconch creatures in Jeffersonville Limestone found in Louisville, Kentucky. The white material might be what is left of their shells or part of the material that made the mode. I found two pieces next to each other but they do not seem to fit together. A heavy piece of earth moving machinery crushed this rock so it is hard to say what it fully looked like.
The rostroconch I have seen found in this area are called Hippocardia.