  Optical spectroscopy in high magnetic fields $B\leq65$ T is used to reveal
the very different nature of carriers in monolayer and bulk transition metal
dichalcogenides. In monolayer WSe$_{2}$, the exciton emission shifts linearly
with the magnetic field and exhibits a splitting which originates from the
magnetic field induced valley splitting. The monolayer data can be described
using a single particle picture with a Dirac-like Hamiltonian for massive Dirac
fermions, with an additional term to phenomenologically include the valley
splitting. In contrast, in bulk WSe$_{2}$ where the inversion symmetry is
restored, transmission measurements show a distinctly excitonic behavior with
absorption to the 1s and 2s states. Magnetic field induces a spin splitting
together with a small diamagnetic shift and cyclotron like behavior at high
fields, which is best described within the hydrogen model.
