  The Ursa Major Cluster appears to be unevolved and made up of HI-rich spiral
galaxies. B,R,I,K' photometry has been obtained for 79 galaxies, including 62
in a complete sample with M(B)<-16.5 mag. The K' information is particularly
important for the present discussion because it is not seriously affected by
obscuration. There is reasonably convincing evidence that the distribution of
exponential disk central surface brightnesses is bimodal. Disks avoid the
domain between the high and low surface brightness zones. The few intermediate
surface brightness examples in the sample all have significant nearby
neighbors. The high surface brightness (HSB) galaxies exhibit a range
-21<M(B)<-17 while the low surface brightness (LSB) galaxies are found with
-19<M(B) down to the completion limit. HSB and LSB galaxies in the overlap
regime -19<M(B)<-17 that are indistinguishable in luminosity-line width plots
have very distinct locations in surface brightness-scale length plots. The
existence of separate HSB and LSB families suggests that there are discrete
radial configurations that are stable. Galaxies are driven into one of these
regimes. The HSB state has a lower luminosity cutoff. It is likely that the HSB
galaxies are dominated by dissipational matter at their centers while the LSB
galaxies are dark matter dominated. The HSB family subdivides into those with,
and without, substantial bulges. Evidently, there are two thresholds probably
controlled by angular momentum content or transfer. Passing from high to low
specific angular momentum, there is first the transition from LSB to HSB
regimes, then the transition from exponential disk to disk plus bulge regimes.
