  The Dirac equation is extended for a relativistic electron in an
orthorhombically-anisotropic conduction band. Its covariance is established
with general proper and improper Lorentz transformations. In the
non-relativistic limit, the kinetic and Zeeman energy terms of the Hamiltonian
are both determined by the same three effective masses, and the quantum spin
Hall effect is derived. This has important consequences for magnetic
measurements of many classes of clean anisotropic semiconductors, metals, and
superconductors. The Zeeman energy is vanishingly small for magnetic fields
parallel to clean monolayers and in all directions in quasi-one-dimensional
materials.
