  It is demonstrated that the 3--vector $\bs{S}$ currently associated to the
spin in an inertial frame does not contract, but rather dilates, in the
direction of the velocity. The correct vector $\bs{T}$ is individuated. The
equation of motion for the two vectors is shown to contain two terms, a common
linear rotation, identified with Thomas precession, and also a nonlinear
rotation depending on the direction of the spin itself.
