  I give analytic expressions for the relative number, luminosity, and mass
density of disc galaxies as a function of surface brightness. These surface
brightness distributions are asymmetric, with long tails to lower surface
brightnesses. This asymmetry induces systematic errors in most determinations
of the galaxy luminosity function. Galaxies of low surface brightness exist in
large numbers, but the additional contribution to the integrated luminosity
density is modest, probably 10 -- 30%.
