Systematic revision of Palaeocene brackish water Gastropoda from Mons, Belgium, based on their early ontogenetic shells

The systematic position of Early Mid-Palaeocene brackish water Gastropoda from the Dano-Montian of Mons, South-east Belgium, is revised. Representatives of the subclasses Neritimorpha (with the new genus Monsneritina - Neritoidea, Neritidae). Caenogastropoda (with the new genus Monspotamides - Cerithioidea, Potamididae) and Heterostropha are characterised with regard to their paleoecology. Three different modes of early ontogenetic development are recognised and paleoecological inferences are discussed.