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.Peel J.S. 1991. Functional morphology of the Class Helcionelloida nov., and the early evolution of the Mollusca. In Simonetta A.M. & Conway Morris S. The early evolution of metazoa and the significance of problematic taxa. Cambridge University Press, pp. 157–177. 

The latest word on this group is the classification of Parkhaev, who splits it into three subclasses.P. Yu. Parkhaev (2006) "Adaptive radiation of the Cambrian helcionelloid mollusks (Gastropoda, Archaeobranchia). " In: S.V. Rozhnov (ed.) "Evolution of the biosphere and biodiversity. Towards the 70th anniversary of A. Y. Rozanov". 2006. Moscow, pp. 282-296.

 References 

Category:Molluscs