Original Description Description. Test is biserial throughout. Proloculus is small and followed by larger second chamber. Chambers are subglobular to globular in early portion of test and often reniform in adult. Periphery is broadly rounded. Aperture is a low- to medium-high arch situated at base of last-formed chamber; it is bordered by two symmetrically developed flanges. Chamber surface is smooth or with fine longitudinal costae in earlier species and completely costate in evolved ones. Test wall is calcitic, hyaline, microperforate to finely perforate. Pores are situated in intercostal spaces or they might penetrate and interrupt costae. Size: Etymology: The Latin prefix plano (= plane) is added to the pre-existing genus name Heterohelix. Extra details from original publication Remarks. Planoheterohelix is the first genus of Cretaceous serial planktic foraminifers with completely symmetrical tests as seen in edge view. Test symmetry differentiates Planoheterohelix from its ancestor Protoheterohelix, which displays asymmetrical tests or periapertural structures. Late Albian–Cenomanian species of Planoheterohelix have a microperforate test wall (0.5–0.9 mm); finely perforate species are known only from higher stratigraphic levels (Turonian–lower Maastrichtian).
Planoheterohelix moremani (Cushman), 1938, p. 10, pl. 2, figs. 1–3;
Planoheterohelix postmoremani n. sp.;
Planoheterohelix papula (Belford), 1960, p. 57, pl. 15, figs. 6–8, text-fig. 3:1–6;
Planoheterohelix planata (Cushman), 1938, p. 12, pl. 2, figs. 13–14;
Planoheterohelix rumseyensis (Douglas), 1969, p. 159, pl. 11, figs. 9–11.
The latter three species occur at higher stratigraphical levels (Coniacian–lower Maastrichtian).