  We propose a phenomenological model for pattern formation in a vertically
vibrated layer of granular material. This model exhibits a variety of stable
cellular patterns including standing rolls and squares as well as localized
objects (oscillons and worms), similar to recent experimental
observations(Umbanhowar et al., 1996). The model is an amplitude equation for
the parametrical instability coupled to the mass conservation law. The
structure and dynamics of the solutions resemble closely the properties of
localized and cellular patterns observed in the experiments.
