Zygodiscus amphipons Bramlette and Martini, 1964
Loxolith is constructed of a single or double wall together with a diagonal cross that may support a distal process. The morphology, structural organisation, and optical behaviour of the central cross provide key characters for distinguishing among species.
Single‑ or double‑walled structures are typically in constant extinction under crossed polars, although they may occasionally display whitish‑grey interference colours. The optical behaviour of the central cross varies according to its morphology and structural organisation.
The type species of Helicolithus is also double‑walled, exhibiting constant extinction under crossed polars in plan view. Reinhardt & Górka (1967), in their description of the lectotype, noted that “the rims are indistinct between crossed nicols,” and the published images of the lectotype support this observation. Strictly speaking, Helicolithus is therefore a senior synonym of Chiastozygus.
In practice, however, Helicolithus has been applied to forms possessing a double wall with a birefringent inner wall and a cross. In this study, species with a protolith wall composed of eight birefringent segments are placed in a new genus, Aktasia, whereas species with a zeugoid inner wall exhibiting birefringence are retained within Helicolithus.
Species concepts for Helicolithus and Chiastozygus vary substantially among authors, and a comprehensive revision of both genera is still needed.
Bramlette, M. N. & Martini, E. 1964. The great change in calcareous nannoplankton fossils between the Maastrichtian and Danian. Micropaleontology. 10(2): 291-322.
Gartner, S., 1968. Coccoliths and related calcareous nannofossils from Upper Cretaceous deposits of Texas and Arkansas. University of Kansas Paleontological Contributions, Articles 48 (Protista 1), 1-56.
Reinhardt, P. & Górka, H. 1967. Revision of some Upper Cretaceous Coccoliths from Poland and Germany. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 129(3): 240-256.