  A suitable nonlinear interaction between dark matter with an energy density
$\rho_{M}$ and dark energy with an energy density $\rho_{X}$ is known to give
rise to a non-canonical scaling $\rho_{M} \propto \rho_{X}a^{-\xi}$ where $\xi$
is a parameter which generally deviates from $\xi =3$. Here we present a
covariant generalization of this class of models and investigate the
corresponding perturbation dynamics. The resulting matter power spectrum for
the special case of a time-varying Lambda model is compared with data from the
SDSS DR9 catalogue. We find a best-fit value of $\xi = 3.25$ which corresponds
to a decay of dark matter into the cosmological term. Our results are
compatible with the $\Lambda$CDM model at the 2$\sigma$ confidence level.
