The onset of convection in a polytropic atmosphere with rotation and magnetic field is considered in a geometry such that rotation, magnetic field and gravity are mutually perpendicular. The anelastic approximation is used together with the low diffusion magnetogeostrophic approximation. The factors determining the pattern of convection are discussed, with particular reference to the question of whether convection rolls are primarily aligned with the rotation axis or the magnetic field. As in the Boussinesq problem, the global Elsasser number ℬ2/2μρΩη plays a key role.