  It is shown that the dynamics of a two-dimensional crystal with a finite
concentration of dislocations, as well as vacancy and interstitial defects, is
governed by the hydrodynamic equations of a viscoelastic medium. At the longest
length scales the viscoelasticity is described by the simplest Maxwell model,
whose shear and compressional relaxation times are obtained in terms of
microscopic quantities, including the density of free dislocations. At short
length scales, bond orientational order effects become important and lead to
wavevector dependent corrections to the relaxation times.
