  We investigate the optical transition selection rules for excitonic Rydberg
series formed in massive Dirac cones. The entanglement of the exciton envelop
function with the pseudospin texture leads to anomalous selection rules for
one-photon generation of excitons, where $d$-orbitals can be excited with the
opposite helicity selection rule from the $s$-orbitals in a given valley. The
trigonal warping effects in realistic hexagonal lattices further renders more
excited states bright, where $p$-orbitals can also be accessed by one-photon
excitation with the opposite valley selection rules to the $s$-orbitals. The
one-photon generation of exciton in the various states and the intra-excitonic
transition between these states are both dictated by the discrete in-plane
rotational symmetry of the lattices, and our results show that in hexagonal 2D
materials the symmetry allowed transitions are enabled when trigonal warping
effects are included in the massive Dirac fermion model. In monolayer
transition metal dichalcogenides where excitons can be generated by visible
light and intra-excitonic transitions can be induced by infrared light, we give
the strength of these optical transitions, estimated using modified
hydrogen-like envelope functions combined with the optical transition matrix
elements between the Bloch states calculated at various $k$ points.
