Kokia borealis Perch-Nielsen, 1988
Varol & Girgis (1994) suggested that Kokia is possibly an ascidian spicule, mainly based on the morphological similarities between the weakly birefringent Kokia curvata and the didemnid ascidian spicules (all has axial c-axis orientation).
However, the type species Kokia borealis is non-birefringent and morphologically similar to some spicules of Neanthozoites/Cyctodytes produced by Polycitoridae (unfortunately, we don't have information about their optical properties).
Perch-Nielsen, K., 1988. New Lower Cretaceous calcareous nannofossil species from England. INA Newsletter. 10(1), 30-37.