  We model the cosmic medium as the mixture of a generalized Chaplygin gas and
a pressureless matter component. Within a neo-Newtonian approach (in which,
different from standard Newtonian cosmology, the pressure enters the
homogeneous and isotropic background dynamics) we compute the matter power
spectrum. The 2dFGRS data are used to discriminate between unified models of
the dark sector (a purely baryonic matter component of roughly 5 percent of the
total energy content and roughly 95 percent generalized Chaplygin gas) and
different models, for which there is separate dark matter, in addition to that
accounted for by the generalized Chaplygin gas. Leaving the corresponding
density parameters free, we find that the unified models are strongly
disfavored. On the other hand, using unified model priors, the observational
data are also well described, in particular for small and large values of the
generalized Chaplygin gas parameter $\alpha$. The latter result is in agreement
with a recent, more qualitative but fully relativistic, perturbation analysis
in Gorini et al.
