  We review different scenarios for the motion and merging of Dirac points in
two dimensional crystals. These different types of merging can be classified
according to a winding number (a topological Berry phase) attached to each
Dirac point. For each scenario, we calculate the Landau level spectrum and show
that it can be quantitatively described by a semiclassical quantization rule
for the constant energy areas. This quantization depends on how many Dirac
points are enclosed by these areas. We also emphasize that different scenarios
are characterized by different numbers of topologically protected zero energy
Landau levels
