Phylum Mollusca Linnaeus, 1758 - molluscs Conchifera Gegenbaur, 1878 see Subphylum Conchifera Gegenbauer, 1878 Cyrtosoma Class Gastropoda Cuvier, 1795 - snails and slugs Subclass †Archaeobranchia Parkhaev, 2001 Order †Helcionelliformes Golikov & Starobogatov, 1975 1 Superfamily †Yohelcionelloidea nom. null. see Superfamily Yochelcionelloidea Runnegar & Jell, 1976 2 Superfamily †Helcionelloidea Wenz, 1938 see Superfamily Scenelloidea Miller, 1889