The western Australian form has a row of well marked nodules rather than ridges and an unindented lip while the southern Australian form has much less distinct ridges and a nearly smooth lip. At one time it was thought that these different forms were distinct species but it has now been established that the variations in sculpture are a function of different living conditions. Specimens gathered from New South Wales with deep ridges were kept in a still water aquarium for three years during which time the new shell growth was at first shallowly indented and later was smooth.