The dispersion relation of the magnetic excitations of the hexagonal ions in DHCP Pr and the selection rules for the linear coupling to the phonons are determined by general symmetry considerations. The magnetic excitations propagating in the symmetry directions are considered in the cases of an external magnetic field applied along an a and a b direction. The magnetic excitations are approximated by pseudo-boson excitations of the spin sub-space, J=4, MJ=0 and +or-1, and the presence of the ions on the cubic sites is neglected. The selection rules deduced agree with experimental observations. The experimental result for the strength of the exciton-phonon interaction is used in an estimate of the effects of an applied field on the elastic constants of Pr at zero temperature.