5.1. The A-Systems
The spectral types in the region

3850-4100 are of class A,
and hence we shall refer to them as "A-systems"; they are listed
in Table I.
| NGC | Hubble |
| 672 | SBc |
| 1518 | Scp |
| 3319 | SBc |
| 3389 | Sc |
| 4449 | Irr |
| 4559 | Sc |
| 4631 | Sc |
These seven galaxies are of three kinds in Hubble's classification:
Irr, SBc, and Sc. They seem to have similar stellar populations,
and the principal contributors to the total light in the violet
region are main-sequence stars of type A. This classification
appears to be unique, for the observed broad hydrogen absorption
lines could not be produced by a mixture of B- and F-type stars.
The spectral type for all of these systems is around F8 at
4340;
therefore the degree of compositeness is considerable.