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K. Hatano, D. Branch, E. Baron (U. Oklahoma), P. Garnavich (CfA)
Certain aspects of the spectra of Type~Ia supernovae, such as the ``Si~II ratio" obtained from the relative strengths of absorption features near 5800 and 6150~Å\ (attributed primarily to Si~II \lambda5972 and \lambda6355), have been shown to correlate very well with the light--curve decline--rate parameter \Deltam15 (Nugent et~al. 1995, ApJ, 455, L147); this suggests that the concept of a one--dimensional spectroscopic sequence that parallels the photometric sequence is useful. However, by now enough high quality data have become available to show that SN~Ia spectra do not all fit into a one--dimensional sequence. We demonstrate this by plotting the velocity corresponding to the blueshift of the 6150~Å\ absorption minimum, 10 days after the time of maximum brightness, against the Si~II index (or \Deltam15). There is no simple relationship between the 6150~Å\ blueshift and the Si~II index (or \Deltam15) among spectroscopically normal SNe~Ia. We also show that when spectra of various SNe~Ia obtained about a week before maximum brightness are arranged according to expansion velocity, they form a sequence that resembles the pre--maximum--brightness time evolution of the well observed SN~1994D. Several possible interpretations of these findings are discussed.