  The paper deals with the 2-D system of gas dynamics without pressure which
was introduced in 1970 by Ya.Zeldovich to describe the formation of large-scale
structure of the Universe. Such system occurs to be an intermediate object
between the systems of ordinary differential equations and hyperbolic systems
of PDE. The main its feature is the arising of singularities: discontinuities
for velocity and delta functions of various types for density. The rigorous
notion of generalized solutions in terms of Radon measures is introduced and
the generalization of Rankine-Hugoniot conditions is obtained. On the basis of
such conditions it is shown that the variational representation for the
generalized solutions, which is valid for 1-D case, in 2-D case generally
speaking does not take place. A nontrivial 1-D system of nonstrictly hyperbolic
type is also obtained to describe the evolution inside the shock.
