  A comparative study of optical spectra of Type Ia supernovae (SNe Ia) is
extended, in the light of new data. The discussion is framed in terms of the
four groups defined in previous papers of this series: core normal (CN); broad
line (BL); cool (CL); and shallow silicon (SS). Emerging features of the SN Ia
spectroscopic diversity include evidence (1) that extreme CL SN 1991bg-likes
are not a physically distinct subgroup and (2) for the existence of a
substantial number of SN 1999aa-like SSs that are very similar to each other
and distinguishable from CN even as late as three weeks after maximum light. SN
1999aa-likes may be relatively numerous, yet not a physically distinct
subgroup. The efficacy of quantitative spectroscopic subclassification of SNe
Ia based on the equivalent widths of absorption features near 5750 A and 6100 A
near maximum light is discussed. The absolute magnitude dispersion of a small
sample of CNs is no larger than the characteristic absolute magnitude
uncertainty.
