Helcionelloida

The Helcionelloida are an extinct class of fossil molluscs.
They are among the earliest animals with mineralised shells. They are also the oldest known molluscs wth a shell. Some members of this class were mistaken for Monoplacophorans. The class was erected by Peel in 1991.
The latest word on this group is the classification of Parkhaev, who splits it into three subclasses.