Limpet is a common name applied to aquatic snails with shells broadly conical in shape, rather like the conical Asian hat. The term "limpet" is purely informal, a term of convenience; it refers to any gastropod whose shell has no obvious coiling such as one sees in familiar garden snails or in winkles. Although all limpets are Gastropoda, the group is highly polyphyletic, meaning that the various lines that we call limpets have descended independently from different ancestral Gastropoda. This general category of conical shell is technically known as "patelliform", meaning dish-shaped. Some species of limpet live in fresh water, but by far the majority are marine. All members of the large and ancient marine clade Patellogastropoda are limpets, and within that clade the family Patellidae in particular often are called the "true limpets". However, other groups, not in the same family, also are called limpets of one type or another because of the general shapes of their shells. Examples include the Fissurellidae; they are the keyhole limpet family, contained in the clade Vetigastropoda, though many of the members of the Vetigastropoda do not have the morphology of limpets at all.