Simply put

'What's this?'

'It's a marble that has the shells of early relatives of the ammonites all through it. These are the shells here... Most fossils are found in limestones, and marble is a limestone that's been metamorphosed under high temperatures and pressures. Usually the rock's deformed by this process and any structures it contained, like fossils or bedding planes are lost. In this case, though, it's probably only been heated and the fossil shapes are still intact. They're called orthocone nautiloids. The orthocone part means straight-shelled and the black and white stripy parts - here, here and here - through the stone are the chambered shells filled with white calcite crystals. When the animals were alive these chambers would have been filled with a mixture of liquid and gas and the nautiloids could regulate their buoyancy by adjusting the mix. They had tentacles that protruded from the front of their shell and their body, which would have been fairly soft and flexible, was protected by the extended first chamber. Its later relatives, ammonites, mostly had spiral shells instead of straight and looked very similar to today's nautilus. They now think octopuses are the most closely linked, though.'

'But - what are they? Are they fish?'

'A squid in a cone.'

'Oh, riiiight. Cool.'