  We present the hydrodynamic theory of coherent collective motion ("flocking")
at a solid-liquid interface, and many of its predictions for experiment. We
find that such systems are stable, and have long-range orientational order,
over a wide range of parameters. When stable, these systems exhibit "giant
number fluctuations", which grow as the 3/4th power of the mean number. Stable
systems also exhibit anomalous rapid diffusion of tagged particles suspended in
the passive fluid along any directions in a plane parallel to the solid-liquid
interface, whereas the diffusivity along the direction perpendicular to the
plane is not anomalous. In the remaining parameter space, the system becomes
unstable.
