Although these specimens look like the shells of molluscs or ammonites they are in fact the calcareous armour secreted by worms. These most likely belong to the genera Rotularia from the Cretaceous but similar types of worms have been found encrusting mollusc shells back to the Ordovician.

This is an example of a Jurassic worm, probably a Serpula sp., encrusting the shell of Gryphaea. This specimen comes from Lyme Regis