Well, Tom, Identifying sponges by email is always perilous, but since sponges are common and sponge experts are rare, I'll chance it. Various species of Cliona often encrust shells and also bore into them. The borings start out as a radiating system of small, round cavities connected to one another (and to the surface of the shell) by short, narrow galleries. Later, the system develops into a network that may be more or less dense depending on the species of sponge. Cliona may grow on living oysters, but attacks only the dead shell and not the inner surface, at least until the mollusk has died. It is therefore common to see shells that have a clean inner surface and a riddled outer surface. The name Cliona is similar to Clione and Ciona, which refer to other marine animals. The names are sometimes confused in the literature. Andrew K. Rindsberg Geological Survey of Alabama "Diagnosis at a Distance"