Polarised light micrograph of fossil shells belonging to the order known as Foraminifera. It comprises unicellular protozoans whose shells form an important component of chalk and of many deep sea oozes. They live in the sea or salt-water lakes. Their shells are composed of several chambers which are usually penetrated by pores through which the cellular contents (protoplasm) are extruded. In previous geological ages they occurred in such enormous numbers that their shells have formed immense deposits. The shells shown here are made of calcium carbonate. Magnification: x50 at 35mm size.