  We examine the validity of the recently proposed semi-Poisson level spacing
distribution function P(S), which characterizes `critical quantum chaos', in 2D
disordered systems with spin-orbit coupling. At the Anderson transition we show
that the semi-Poisson P(S) can describe closely the critical distribution
obtained with averaged boundary conditions, over Dirichlet in one direction
with periodic in the other and Dirichlet in both directions. We also obtain a
sub-Poisson linear number variance $\Sigma_{2}(E)\approx \chi_{0}+ \chi E$,
with asymptotic value $\chi\approx0.07$. The obtained critical statistics,
intermediate between Wigner and Poisson, is relevant for disordered systems and
chaotic models.
