  We analyze the low-energy properties of two-dimensional direct-gap
semiconductors, such as for example the transition-metal dichalcogenides
MoS$_2$, WS$_2$, and their diselenide analogues MoSe$_2$, WSe$_2$, etc., which
are currently intensively investigated. In general, their electrons have a
mixed character -- they can be massive Dirac fermions as well as simple
Schr\"odinger particles. We propose a measure (Diracness) for the degree of
mixing between the two characters and discuss how this quantity can in
principle be extracted experimentally, within magneto-transport measurements,
and numerically via ab initio calculations.
