Figs 15–20. Calciarcus alaskensis aff. from Antarctica (Figs 15–17: EPOS; Figs 18–20: AMERIEZ); TEM (Figs 15, 18–20) and LM (Figs 16–17) whole mounts. 15 – pair of dividing cells; 16–17 – light micrographs (Nomarski optics) showing cells that are tentatively identified as C. calciarcus; 18 – whole cell collected during the AMERIEZ cruise lying next to a pair of choanoflagellate costal strips. Details from this coccosphere are shown at higher magnification in Figs 19, 20; 19 – high magnification of euhedral crystallites which piled on top of each other form part of a superstructure strut. The black crystallite towards the bottom of the picture is tilted and shows that one dimension of the rhombohedra is significantly shorter than the others; 20 – detail of a cluster of coccoliths in which the superstructure to the left clearly displays its three struts. All superstructures shown are terminated by a blunt spine (arrows) formed by a dense aggregation of crystallites.