  We study background dynamics and the growth of matter perturbations in the
extended quasidilaton setup of massive gravity. For the analysis of
perturbations, we first choose a scalar field matter component and obtain the
conditions under which all scalar perturbations are stable. We work in unitary
gauge for the matter field, which allows us to directly map to known results in
the limit of general relativity. By performing a parameter search, we find that
the perturbations are unstable in general, while a particular choice of
potential, where the scalar field effectively behaves like pressureless matter,
allows for stable perturbations. We next consider the growth of matter
perturbations in a cold dark matter-dominated Universe. Working in conformal
Newtonian gauge, we obtain evolution equations for various observables
including the growth factor and growth rate, and find scale-independent growth
in the quasistatic and subhorizon approximations. We finally show how the
Hubble parameter and matter perturbations evolve in massive gravity for a
specific choice of parameter values, and how this evolution compares to the
standard cosmological model consisting of a cosmological constant and cold dark
matter.
