Turbinidae, the turban snails, are a family of small to large marine gastropod molluscs in the superfamily Trochoidea. Description Turbinidae have a strong, thick calcareous operculum readily distinguishing them from the somewhat similar Trochidae or top snails, which have a corneous operculum. This strong operculum serves as a passive defensive structure against predators that try to enter by way of the aperture or that would break the shell at the outer lip. These operculum are rounded ovals that are flat with a swirl design on one side and domed on the other. They are known as Pacific cat's eye or Shiva eye shells or mermaid money, and are used for decorative purposes. Etymology The common name turban snail presumably refers to the shell's similarity in appearance to a turban. However, the scientific name Turbinidae is based on the genus name Turbo, which is Latin for spinning top, a child's toy. The word turbine has a similar derivation. Taxonomy Previously they were classified in the subclass Prosobranchia, in the order Archaeogastropoda in the superfamily Trochacea. Trochaecea is now a synonym for the superfamily Trochoidea. However, this is a quite ancient group of gastropods, probably originating in the Permian period 298 to 250 million years ago. They have typical primitive characters like the nacreous interior of the shell. 2005 taxonomy Turbinidae belongs to superfamily Turbinoidea according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005). This family consists of eight following subfamilies (according to the taxonomy of the Gastropoda by Bouchet & Rocroi, 2005): Turbininae Rafinesque, 1815 - synonyms: Senectinae Swainson, 1840; Imperatorinae Gray, 1847; Astraliinae H. Adams & A. Adams, 1854; Astraeinae Davies, 1935; Bolmidae Delpey, 1941 Angariinae Gray, 1857 - synonym: Delphinulinae Stoliczka, 1868 Colloniinae Cossmann, 1917 tribe Colloniini Cossmann, 1917 - synonym: Bothropomatinae Thiele, 1924 (inv.); Homalopomatinae Keen, 1960; Petropomatinae Cox, 1960 tribe † Adeorbisinini Monari, Conti & szabo, 1995 tribe † Crossostomatini Cox, 1960 tribe † Helicocryptini Cox, 1960 Moellerinae Hickman & McLean, 1990 † Moreanellinae J. C. Fischer & Weber, 1997 Prisogasterinae Hickman & McLean, 1990 - synonym: "Prisogastrinae" in Bouchet & Rocroi (2005) is a spelling error. Skeneinae W. Clark, 1851 - synonym: Delphinoideinae Thiele, 1924 Tegulinae Kuroda, Habe & Oyama, 1971 2008 taxonomy Turbinidae was redefined and moved to the redefined superfamily Trochoidea according to Williams et al. (2008): Angariidae was elevated to family level, Colloniinae was elevated to family Colloniidae within Phasianelloidea,