  $ $The critical current of a Josephson junction is an oscillatory function of
the enclosed magnetic flux $\Phi$, because of quantum interference modulated
with periodicity $h/2e$. We calculate these Fraunhofer oscillations in a
two-dimensional (2D) ballistic superconductor--normal-metal--superconductor
(SNS) junction. For a Fermi circle the amplitude of the oscillations decays as
$1/\Phi$ or faster. If the Fermi circle is strongly warped, as it is on a
square lattice near the band center, we find that the amplitude decays slower
$\propto 1/\sqrt\Phi$ when the magnetic length $l_m=\sqrt{\hbar/eB}$ drops
below the separation $L$ of the NS interfaces. The crossover to the slow decay
of the critical current is accompanied by the appearance of a 2D array of
current vortices and antivortices in the normal region, which form a bipartite
rectangular lattice with lattice constant $\simeq l_m^2/L$. The 2D lattice
vanishes for a circular Fermi surface, when only the usual single row of
Josephson vortices remains.
