Description: Coccoliths are placoliths with rim structure similar to Calcidiscus, i.e. the V-unit forms distal shield and central-area/tube; R-unit forms proximal shield only. As in the Coccolithaceae, growth occurs downward from the proto-coccolith ring which becomes embedded within the structure so that alternating V- and R-units are only visible on specimens where the proximal shield has broken off.

Distal shield sutures usually show laevogyral curvature. The proximal shield is usually formed of a single layer of elements with sub-radial sutures; sometimes a lower layer is developed, with elements showing strong dextral obliquity (in proximal view). The connection between the proximal and distal shields is weak and they frequently separate.

Genera included:

	Calcidiscus	Circular to broadly elliptical with closed central area or narrow opening	Paleocene-Recent
	Umbilicosphaera	Circular to broadly elliptical with wide central opening	Paleocene-Recent
	Oolithotus	Sub-circular with proximal shield off-centre	Pliocene to Recent
	Hayaster	Nearly flat, with few elements and small proximal shield	Eocene-Recent
	Cryptococcolithus	Ellptical with grill in central area	Miocene
	Ceratolithus HET (old name Neosphaera)	Only one shield, circular. These do NOT belong in the Calcidiscaceae, but they do look rather similar	Pliocene-Recent

NB All these genera were included within the Coccolithaceae in older classifications but were separated by Young & Bown (1987) into the family Calcidscaceae on the grounds of them sharing a simpler coccolith structure than that of the Coccolithaceae. The grouping has been supported by molecular genetic data (Sáez et al., 2003, 2004).

Remarks: Dominant phase of life-cycle non-motile with placolith heterococcoliths. Holococcolith bearing motile phase has been documented in the extant species C. leptoporus and C. quadriperforatus (refs. Gesen et al. 2002, Houdan et al. 2004).





Selected references: Inouye & Pienaar (1984) - cytology of Umbilicosphaera; Kleijne (1993) - detailed description of extant species; Knappertsbusch (2000) Calcidiscus evolution; Young et al. (2004) - coccolith structure.