Galaxies within Clusters
A range of structural types (Abell)
Regular indicates cluster is circular, centrally
concentrated (cf. Globular clusters), and has
mostly elliptical and S0 galaxies.  Can be very
rich with > 1000 galaxies.  Coma is regular.
All others are irregular (e.g., Virgo).
I don’t know why he didn’t just call them type 1
and type 2…!