Bownia mutterlosei

Staurolithites mutterlosei Crux, 1989

In plan view under XPL, the outer wall is in constant extinction because the optic axis (c-axis) is perpendicular to the microscope stage, whereas the inner wall and the cross display white interference colours. The combination of a protolith outer wall and a zeugoid inner wall produces a spiraling effect.

Species of Bownia are characterised by a protolith outer wall, a zeugoid inner wall, and a right-angle cross that is rotated clockwise in distal view and supports a distal process.

Crux, J. A. 1989. Biostratigraphy and palaeogeographical applications of Lower Cretaceous nannofossils from north-western Europe. In: Crux, J. A. & van Heck, S. E. (Eds) Nannofossils and their applications: Proceedings of the 2nd INA Conference, London 1987. British Micropalaeontological Society Publication Series. 143-211.

Varol, O. & Girgis, M. 1994. New taxa and taxonomy of some Jurassic to Cretaceous calcareous nannofossils. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 192: 221-253.