  The a.c. response of an unpinned and finite 2D Wigner crystal to electric
fields at an angular frequency $\omega$ has been calculated in the dissipative
limit, $\omega \tau \ll 1$, where $\tau ^{-1}$ is the scattering rate. For
electrons screened by parallel electrodes, in zero magnetic field the
long-wavelength excitations are a diffusive longitudinal transmission line mode
and a diffusive shear mode. A magnetic field couples these modes together to
form two new magneto-shear modes. The dimensionless coupling parameter $\beta
=2(c_{t}/c_{l})|\sigma_{xy}/\sigma_{xx}|$ where $c_{t}$ and $c_{l}$ are the
speeds of transverse and longitudinal sound in the collisionless limit and
$\sigma_{xy}$ and $\sigma_{xx}$ are the tensor components of the
magnetoconductivity. For $\beta \geqslant 1$, both the coupled modes contribute
to the response of 2D electrons in a Corbino disk measurement of
magnetoconductivity. For $\beta \gg 1$, the electron crystal rotates rigidly in
a magnetic field. In general, both the amplitude and phase of the measured a.c.
currents are changed by the shear modulus. In principle, both the
magnetoconductivity and the shear modulus can be measured simultaneously.
