  Results from the Arecibo HI Strip Survey, an unbiased extragalactic HI
survey, combined with optical and 21cm follow-up observations, determine the HI
Mass Function and the cosmological mass density of HI at the present epoch.
Both are consistent with earlier estimates, computed for the population of
optically selected galaxies. This consistency occurs because, although the
distribution of optical central surface brightnesses among galaxies is flat, we
fail to find a population of galaxies with central surface brightnesses fainter
than 24 B-mag/arcsec^2, even though there is no observational selection against
them.
