On the shape of expanding supershells of neutral hydrogen

The dependence of the shape of the recently discovered giant expanding H I shells on the possible mechanisms of their generation is discussed. It is shown that in the case of a cascade of supernova explosions propagating in the galactic disk all shells with characteristic radii exceeding a critical value Zc must be elongated along the plane of the Galaxy. Objects elongated in the direction perpendicular to this plane or spherical objects with radii exceeding Zc must be due to other mechanisms.