  It is shown that the three-fold rotational symmetry in transition metal
dichalcogenides leads to a Coulomb induced renormalization of the effective
electron and hole masses near the $K$-points of the Brillouin zone. The
magnitude of the renormalization depends on the dielectric configuration. The
effective exciton mass $m=0.4 m_0$ of a freely suspended MoS$_2$ monolayer
changes to $m= 0.35 m_0$ with hBN encapsulation. The mass renormalization
increases the excitonic binding energy and reduces the exciton diamagnetic
shift and cyclotron frequency. Detailed comparisons with high field
measurements of the excitonic diamagnetic shift show excellent agreement.
