A group of unicellular, biflagellate, golden-brown algae characterized by a covering of extremely small (1–35 micrometers) interlocking calcite (the hexagonal form of calcium carbonate) plates called coccoliths. The plates show extinction crosses in a polarizing light microscope; however, detailed study requires the use of electron transmission or electron scanning microscopes. The Coccolithophorida are usually considered plants but possess also some animal characteristics. Botanists assign them to the class Haptophyceae (based on the possession of a haptonema, a threadlike organ of attachment) of the phylum Chrysophyta, and zoologists to the class Phytamastigophorea, superclass Mastigophora, subphylum Sarcomastigophora of the phylum Protozoa.