supershells of neutral hydrogen in the plane of the sky in different velocity channels. A broadening of the 21
cm line due to chaotic motions in the gas and the finite angular resolution of the radio telescope are taken into
account. Taking account of the broadening of the spectral line proves to be of fundamental importance in mod-
eling observations of shells. Criteria usually applied in the preparation of catalogs of regions deficient in HI in
nearby galaxies are analyzed, using several model supershells for the galaxies M31 and HoII as examples.
Itisshown that three of these criteria—the observation of a “hole” in no fewer than three neighboring velocity
channels, immobility of the center of the “hole,” and a sharp contrast between the “hole” and the background
in all relevant channels—are well satisfied for supershells located close to the galactic node line, and are more
poorly satisfied for supershells located far from it.