Possible Phosphatic Nodule with Fossil Sponge

This was found on the beach below the chalk cliffs at Samphire Hoe, Dover. I am not really sure what it is, save that it is very distinct from the usual flints found there. It has been suggested that it is a phosphatic nodule (as opposed to a silicifed flint), and on cutting it in half, it contained a distinct mass of fossil sponge material.

4cm