Aquatic snail

Aquatic snail

Observation - Aquatic snail. Description: A snail shell, looking a bit like an ammonite, found in the very black silt after our pond was dredged. 3 cm across, grey/dark brown. ID?

A snail shell, looking a bit like an ammonite, found in the very black silt after our pond was dredged. 3 cm across, grey/dark brown. ID?