  Hydrogenic excited states of a 2D exciton are degenerate. In the presence of
a weak magnetic field, the $S$-states with a zero momentum of the center of
mass get coupled to the $P$-states with finite momentum of the center of mass.
This field-induced coupling leads to a strong modification of the dispersion
branches of the exciton spectrum. Namely, the lower branch acquires a shape of
a "mexican hat" with a minimum at a finite momentum. At certain magnetic field,
exciton branches exhibit a linear crossing, similarly to the spectrum of a 2D
electron in the presence of spin-orbit coupling. While spin is not involved,
degenerate $S$ and $P$ states play the role of the spin projections. Lifting of
degeneracy due to diamagnetic shifts and deviation of electron-hole attraction
from purely Coulomb suppresses the linear crossing.
