Discoasteraceae
Discoaster Tan Sin Hok 1927
Asteroliths exhibit fascinating optical properties. In plan view, their stellate or rosette-shaped segments appear in constant extinction (dim) under XPL, with the optic axis vertical relative to the microscope stage. In this orientation, they go dim (extinction position) when their vibration directions align with the polariser and analyser (extinction angle = 0°). With the exception of a few species, including Discoaster pentaradiatus, which exhibits interference colours even in the plan view. Furthermore, they demonstrate length-slow (+) elongation. However, when observed from the side, these asteroliths display interference colours due to birefringence.
Tan Sin Hok. 1927a. Discoasteridae incertae sedis. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen. Sect Sci, 30: 411-419.
