With fossils, we usually have the animal or plant itself, though turned to stone.
Sometimes, we get a mould, which is either:
- an external mould: a depression in the rock from which the fossil has fallen out, or
- an internal mould: where the creature/plant contained a cavity, which became filled with sediment (later turned to stone), then the fossil itself was lost (perhaps dissolved), leaving just the sediment in the form of the inside of the creature/plant.
Sometimes we get a cast, which is the result of an external mould becoming filled with sediment, turned to rock, then coming loose from the rock. Casts have the same shape as the original fossil, but none of the internal structure.
You gastropod is an unusual mix of fossil and internal mould.
When the fossil fell out of the rock, it would have been largely intact; comprising a shell and sediment filling the cavity inside. Erosion has removed most of the shell, so we see mostly the cast inside. The shell is preserved just between the spirals, protected from erosion.
It is a nice little thing; the rounding makes it look like it has been cherished and rubbed by your hands for many years.