  We report on an infrared magneto-spectroscopy study of Pb$_{1-x}$Sn$_x$Se, a
topological crystalline insulator. We have examined a set of samples, all in
the inverted regime of electronic bands, with the tin composition varying from
$x=0.2$ to $0.33$. Our analysis shows that the observed response, composed of a
series of interband inter-Landau level excitations, can be interpreted and
modelled using the relativistic-like Hamiltonian for three-dimensional massive
Dirac electrons, expanded to include diagonal quadratic terms that impose band
inversion. In our data, we have not found any clear signature of massless
electron states that are present on the surface of Pb$_{1-x}$Sn$_x$Se crystals
in the inverted regime. Reasons for this unexpected result are discussed.
