km s$^{-1}$ velocity width. Clearly more sensitive followup in the radio line, as well as far infrared and optical observations, would quickly show the distinction.

Some questions remain about the slope of the luminosity function for galaxies at the faint end, and this uncertainty propagates into the faint galaxy tail of the HI-mass function as well. Recent HI surveys have produced values for $\gamma$ (as defined in Eq.(\[himassfnct.eqn\])) in the range 1.2 to 1.7 (Zwaan et al 1997, Schneider 1997), although there is also observational evidence for a sharp decline in the number density of HI rich objects at faint HI luminosity (Hoffman et al 1992), at least in the Virgo cluster. Figure \[loZconfus.fig\] implies that surveys with sensitivities of $\sim$1 mJy or better may be likely to detect OH masers (at $z_{OH}\approx 0.17$) in comparable numbers to the rate at which they detect weak HI signals in the velocity range $v_{HI}< 500$ km s$^{-1}$.

This regime can be explored in more detail by estimating the form of the HI mass function that the OH masers would imitate. Figure \[sketch.fig\] is an aid to visualize the problem. It shows a volume $dV_{OH}$ at $z_{OH}$ that is mapped into the volume 