A division comprising mainly unicellular biflagellate *algae
most of which have a *haptonema
between the two flagella. It contains a single class, the Haptophyceae with many genera that used to be included in the *Chrysophyta
. The new division was recognized on the basis of evidence from electron-microscope studies, which revealed certain characteristic features, such as the presence of scales covering the cells. Certain marine forms, the coc-colithophorids, have small intricate plates (coccoliths) of calcium carbonate completely covering the cell. Cretaceous chalks contain the remains of vast numbers of such algae.