percent composition is iron in the form of goethite. Goethite has the chemical formula of FeO(OH) and belongs to a group known as oxy-hydroxides. There exists amorphous silica between the goethite crystals; surrounding the goethite is a matrix of chitin. Chitin has a chemical formula of C8H13O5N. Other metals have been shown to be present with the relative percent compositions varying on geographic locations. The goethite has been reported to have a volume fraction of approximately 80%. Regional dependency Limpets from different locations were shown to have different elemental ratios within their teeth. Iron is consistently most abundant however other metals such as sodium, potassium, calcium, and copper were all shown to be present to varying degrees. The relative percentages of the elements have also been shown to differ from one geographic location to another. This demonstrates an environmental dependency of some kind; however the specific variables are currently undetermined. Phylogeny Gastropods that have limpet-like or patelliform shells are found in several different clades: Clade Patellogastropoda, example Patellidae, the true limpets, all marine, in five living families and two fossil families Clade Vetigastropoda, examples Fissurellidae, (the keyhole limpets and slit limpets), and Lepetelloidea, small deepwater limpets Clade Neritimorpha, example Phenacolepadidae, small limpets related to nerites Clade Heterobranchia, group Opisthobranchia, example Tylodinidae, the umbrella slugs with a limpet-shaped shell Clade Heterobranchia, group Pulmonata, examples Siphonariidae, Latiidae, Trimusculidae, all air-breathing limpets Other limpets Marine The hydrothermal vent limpets – Neomphaloidea and Lepetodriloidea The hoof snails – Hipponix and other Hipponicidae Slipper snails – Crepidula species, which are sometimes known as slipper limpets Freshwater The pulmonate river and lake limpets – Ancylidae Some species of limpet live in fresh water, but these are the exception. Most marine limpets have gills, whereas all freshwater limpets and a few marine limpets have a mantle cavity adapted to breathe air and function as a lung (and in some cases again adapted to absorb oxygen from water). All these kinds of snail are only very distantly related. In culture and literature Many species of limpets have historically been used, or are still used, by humans and other animals for food. Limpet mines are a type of naval mine attached to a target by magnets. They are named after the tenacious grip of the limpet. The humorous author Edward Lear wrote "Cheer up, as the limpet said to the weeping willow" in one of his letters. Simon Grindle