  In this paper, we revisit the two theoretical approaches for the formulation
of the tachyonic Dirac equation. The first approach works within the theory of
restricted relativity, starting from a Lorentz invariant Lagrangian consistent
with a spacelike four-momentum. The second approach uses the theory of
relativity extended to superluminal motions and works directly on the ordinary
Dirac equation through superluminal Lorentz transformations. The equations
resulting from the two approaches show mostly different, if not opposite,
properties. In particular, the first equation violates the invariance under the
action of the parity and charge conjugation operations. Although it is a good
mathematical tool to describe the dynamics of a space-like particle, it also
shows that the mean particle velocity is subluminal. In contrast, the second
equation is invariant under the action of parity and charge conjugation
symmetries, but the particle it describes is consistent with the classical
dynamics of a tachyon. This study shows that it is not possible with the
currently available theories to formulate a covariant equation that coherently
describes the neutrino in the framework of the physics of tachyons, and
depending on the experiment, one equation rather than the other should be used.
