Coccoliths

Heterococcoliths are typically circular to elliptical discs or rings constructed from one or more radial arrays (cycles) of elaborate and variably shaped crystal units. This disc or ring is termed the rim and encloses a central area, which may be open, virtually closed or spanned by a variety of structures. Heterococcoliths are formed within the cell (in Golgi vesicles) and then extruded to form a composite exoskeleton, the coccosphere. 

Holococcoliths are typically disc- or dome-shaped and formed of numerous, minute (~0.1µm), equidimensional calcite crystallites of simple, usually rhombic, shape. Compared to heterococcoliths, holococcolith morphologies are relatively similar through time, and there is a less well-defined distinction between rim and central-area structures. Holococcolith formation is not well understood but they tend to form in the motile, life-cycle stage and calcification appears to be extracellular. The fossil record of holococcoliths is sporadic and sparse, because their structure is weaker than the heterococcoliths and the small, modern forms rarely survive in sea floor sediments.