Taxa included:

	Coccolithaceae structure complex with R-units forming both the proximal shield and a centro-distal cycle
	Calcidiscaceae - structure relatively simple, R-units form proximal shield only
	Hymenomonadaceae (neritic with no known fossil record)
	Pleurochrysidaceae (neritic with no known fossil record)

Remarks: The Family Coccolithaceae has often been used for all placoliths not placed in the Noelaerhabdaceae (e.g. Jordan & Kleijne 1994; Jordan & Green 1994). Young & Bown (1997) noted that two major structural types occurred in this group and divided it into two families, the Coccolithaceae and Calcidiscaceae, which they then included in their revised order Coccosphaerales. Following Jordan et al. (2004), the correct name for this order is Coccolithales. The Family Pleurochrysidaceae was also tentatively included in this order on the grounds of coccolith structure. This grouping has subsequently been supported by molecular genetic studies (Edvardsen et al. 2000; Fujiwara et al. 2001; Sáez et al. 2003, 2004) and further studies of the structure of Pleurochrysis coccoliths (Marsh 1999). In addition, the Hymenomonadaceae were included by Young et al. (2003), since numerous cytological and life-cycle characters indicate that they are closely related to the Pleurochrysidaceae (e.g. Gayral 1971; Gayral & Fresnel 1983; Fresnel & Billard 1991; Fresnel & Probert subm.), even though their coccolith structure is not obviously similar. This placement is also supported by molecular genetics (Sáez et al. 2004).

Coccolith structure: The Coccolithaceae, Calcidiscaceae and Pleurochrysidaceae all form placolith coccoliths, i.e. coccoliths formed of two shields separated by a tube. Common structural features are:

1. Growth occurs downward as well as upward from the proto-coccolith ring, which consequently becomes embedded within the rim. Hence, on intact specimens, there is no obvious belt of alternating V- and R-elements, but such a belt is seen on specimens where the proximal shield has been partially detached (Young et al. in press).

2. The V-units form the distal shield and most of the tube, whilst the R-units form the proximal shield and, in some cases, part of the tube. In consequence, the distal shield is dark in cross-polarised light, whilst the proximal shield is bright.

It is also noteworthy that the coccospheres in all cases are monomorphic. This applies even to the motile genera of the Pleurochrysidaceae and Hymenomonadaceae, and to the known holococcolith stages of the Coccolithaceae and Calcidiscaceae.



Life-cycles and culture studies: Heteromorphic life-cycles have been documented from culture studies of the Coccolithaceae, Pleurochrysidaceae and Hymenomonadaceae and inferred from observations of combination coccospheres of Calcidiscaceae (Parke & Adams 1960; Rowson et al. 1986; Fresnel & Billard 1991; Kleijne 1991; Fresnel 1994; Cortes 2000; Renaud & Klaas 2001; Geisen et al. 2002; Billard & Inouye 2004; Houdan et al. 2004). In the Coccolithaceae and Calcidiscaceae, non-motile diploid heterococcolith-bearing stages alternate with motile haploid holococcolith-bearing stages. In the Pleurochrysidaceae and Hymenomonadaceae, motile diploid heterococcolith-bearing stages alternate with motile and benthic haploid non-calcifying stages.