  The electrostatic theory of magnetoplasmons on a semi-infinite magnetized
electron gas is generalized to mixed Faraday-Voigt configurations. We analyze a
new type of electrostatic surface waves that is strongly direction-dependent,
and may be realized on narrow-gap semiconductors in the THz regime. A general
expression for the dispersion relation is presented, with its dependence on the
magnitude and orientation of the applied magnetic field. Remarkably, the group
velocity is always perpendicular to the phase velocity. Both velocity and
energy relations of the found magnetoplasmons are discussed in detail. In the
appropriate limits the known magnetoplasmons in the higher-symmetry Faraday and
Voigt configurations are recovered.
