Model of Nodosaria tetragona, a species of a planktonic foraminifer indigenous to shallower zones of oceanic waters. Foraminifera are single-celled protists sized from under 1 up to 15 mm, having calcareous shells, floating in limitless numbers while alive and settling on the seafloor to form vast deposits of nearly pure organisms or mixed with mud, later to become sedimentary chalk, limestone and marls. They are valuable guide fossils for palaeontologists to establish the age of geological deposits.