  We present scattering cross sections $\sigma_\text{scatt}$ of ultracold
neutrons (UCN) in liquid deuterium at T = 20.6 K, as recently measured by means
of a transmission experiment. The indispensable thorough raw data treatment
procedure is explained. A calculation model for coherent and incoherent
scattering in liquid deuterium in the hydrodynamic limit based on appropriate
physical concepts is provided and shown to fit the data well. The applicability
of the incoherent approximation for UCN scattering in liquid deuterium was
tested and found to deliver acceptable results.
