  We have employed magneto-optical imaging to visualize the occurrence of flux
avalanches in a superconducting film of a-MoGe. The specimen was decorated with
square antidots arranged in a square lattice. We observed avalanches with the
anomalous habit of forming trees where the trunk is perpendicular to the main
axis of the square lattice, whereas the branches form angles of 45 degrees. The
overall features of the avalanches, and in particular the 45 degree direction
of the branches, were confirmed by numerical simulations.
