Cerion and Tudora from the Florida Keys and Bonaire secrete shells composed of prismatic, homogeneous, and crossed-lamellar aragonitic layers. Strontium, magnesium, and sodium contents of shells of Cerion from Sugarloaf Key are greatly influenced by the marine waters that regularly inundate this island. In contrast, the concentrations of these elements in shells in Cerion from Windley Key are controlled by both freshwater, in the form of precipitation, and seawater seeping through the rock into the quarry. Strontium and, to a lesser degree, magnesium are deficient and sodium is relatively high in shells of Cerion and Tudora from dried-out ponds on Bonaire. As with the other elements, concentrations of iron, manganese, and nickel in the shells of the studied gastropods are also controlled by the type and amount of water present in each local environment.