Coccospheres: motile, monothecate, with elongate shape.

Coccoliths: muroliths without flanges, usually termed scapholiths (synonym rhomboliths). The rim is predominantly formed of V-units, with small R-units at the base/inner margin (our obs.). The central-area has a single lath-cycle; pairs of laths from opposite sides of coccolith meet, forming transverse bars. Typically rhombic but there is also an extant form Alveosphaera bimurata with elongate oblong coccoliths. Reference: Manton & Oates (1985) - coccolith structure.

Classification: This family is not recognised in some classifications of the extant coccolithophores, with the genera instead being included in the Syracosphaeraceae, mainly due to similarities between central-area structures. We prefer to maintain it as a separate family since the rim structure is not identical to that of typical Syracosphaeraceae. The group ranges back into the Mesozoic and may have evolved from the Stephanolithiaceae (see e.g. Perch-Nielsen 1985a, Bown & Young 1997). To date there is no molecular genetic data to confirm placing of the family within the Syracosphaerales.

There is a non-calcifying haptophyte Navisolenia Lecal 1965, which has an elongate test, covered in rhombic scales. As discussed by Leadbeater & Morton (1973) and Manton & Oates (1985) it is strikingly similar to Calciosolenia. These similarities include, in addition to basic form, the patterning of the base-plate scale and sense of asymmetry of the scales. Hence, it was included in the Calciosoleniaceae by Jordan and Green (1994) and Jordan et al. (2005). However, rhombic plates are clearly an efficient way to cover an elongate test and this basic morphology has almost certainly evolved separately in the genus Placorhombus, so it may be an unrelated homoeomorph.