fossil marine animal genera Linnaeus, 1758 - animals
   Phylum Mollusca Linnaeus, 1758 - molluscs
     Class Gastropoda Cuvier, 1795 - snails and slugs
       Subclass Opisthobranchia Milne-Edwards, 1848 - opisthobranchs see Order Opisthobranchia Milne-Edwards, 1848
         1 Order Cephalaspida Fischer, 1883
         2 Order Heterostrophia
         3 Order †uncertain
         4 Order Thecosomata Blainville, 1824 - shelled pteropods see Suborder Thecosomata Blainville, 1824
         5 Order Sacoglossa Ihering, 1876 - sacoglossans
         6 Order Anaspidea Fischer, 1883 - sea hares
         7 Order Umbraculomorpha Schmekel, 1985