Recent content by cousin it

A ventral and a dorsal of an asaphid trilobite(Isotelus gigas). Ordovician of Kentucky http://www.flickr.com/photos/30726183@N05/5423389797/

That is not "tafoni" weathering. I agree with the earlier respondent that it is some kind of cnidarian. Take a hammer and give it a good whack... let us see a fresh surface.