Turns out that LSB galaxies can in fact be found, and volume corrections
suggest that they may be as numerous as HSB galaxies - but not in any of
the catalogs! The example of Malin 1: LSB with bright nucleus in direction
of Virgo, but at z=0.08 (Virgo is at 0.004!). Galaxy has CSB=26 and a
scale length of 55 kpc! At Virgo distance, it would be a degree across,
and at Andromeda distance, it would be 20 degrees across!
McGaugh Fig 5
gives an estimate of number of LSB's with the assumption
that and are uncorrelated - which appears to be the case
(McGaugh, Schombert and Bothun Fig 1).