It is pointed out that, in recent publications, several authors have incorrectly omitted a porosity factor, from a term modeling the effect of a magnetic field or rotation, in the momentum equation modeling flow in a porous medium. The error is linked with the way in which the pressure is incorporated in the standard differential form of Darcy＇s law, and this is discussed in detail. A new form for this equation is proposed. Also, the analogy between: (i) Darcy flow in an isotropic porous medium with a magnetic field or rotation effect present; and (ii) flow in a medium with anisotropic permeability, is discussed.