The index—index relation between the two metal line indices used in this paper for the sample of Coma galaxies. A grid of single age stellar population models from Worthey (1994) is overplotted. Solid lines connect points of equal age; dashed lines connect points of equal metal abundance. The figure shows that for absorption strengths of C₂4668>6 Å, the models do not match the parameter space occupied by the observations very well. This is most simply interpreted as an ‘overabundance’ of C₂4668 compared to Fe. The symbols, which are defined in Fig. 1, are coded by galaxy morphology, and residual from the CMR.