Janthina janthina

Helix Janthina | Linnaeus, 1758
Ianthina violacea | Bolten 1795
Ianthina communis | Lamarck, 1799
Ianthina fragilis | Lamarck, 1799
Ianthina bicolour | Menke, 1828

Many other synonyms, see Van der Spoel et al., 1997.