Foraminifera

A type of zooplankton, these are Protozoans that secrete shells (tests) made of calcite (CaCO3). There are two major groups, the planktonic or free floating and the benthic or bottom dwelling which reside on the ocean floor. Their skeletons have chambers separated by a septa (wall). An opening called the foramen connects the chambers with the final opening named the aperture. They are the most important fossil group in both biostratigraphy and paleoenvironmental reconstructions.