Cleistorhabdus williamsii Black 1972a
The distal shield and the segments in the central area remain in constant extinction under crossed polars because the optic axis (c‑axis) is perpendicular to the microscope stage, whereas the proximal shield, the tube cycle, and the central process exhibit first‑order greyish‑white interference colours.
Podorhabdid is constructed of a distal shield composed of non‑imbricated segments, a proximal shield, a tube cycle, and a central area completely filled by segments - leaving no pores - that supports a prominent distal process.
Black, M. 1972a. British Lower Cretaceous Coccoliths. I-Gault Clay (Part 1). Palaeontographical Society Monograph. 126: 1-48.