  We investigate the fine structure of the edge states energy spectrum for
zigzag and armchair ribbons of graphene in a strong magnetic field. At low
energy, the spectra can be described by an effective Schrodinger Hamiltonian
with a double well potential, symmetric in the zigzag case and asymmetric in
the armchair case. We develop a semiclassical formalism based on the WKB
approximation to calculate analytically the energy spectrum for the two types
of edges, including regions which were not studied earlier. Our results are in
very good quantitative agreement with numerical calculations. This approach
leads to a qualitative description of the spectra in terms of the quantization
of unusual classical orbits in the real space.
