Cambrian to Recent. Calcite or aragonite. Some have shells with cemented grains. Very transparent and glassy. Most abundant phytoplankton in the modern ocean.
Circular forms have radial ornamentation.
Elongate forms have transverse markings. They are covered with pores. Planktonic or attached. This image contains twenty five diatoms of various shapes which have been assembled into the shape of a star. The circular diatom in the center of the arrangement is 90 microns in diameter.