1. INTRODUCTION

The purpose of this conference is to debate the usefulness of the morphological classification of galaxies and whether it should be replaced by a scheme based on the physical properties of galaxy types. In either case, one example of where morphological classification has been very successful is in the understanding of first-ranked ellipticals in clusters, i.e. the brightest cluster members (BCM's). In this incidence, I define successful in the sense that the morphological classification of BCM's alerted the astronomical community of their special nature and, with further study, led to a substantial contribution to our understanding of dynamical evolution. In this review I will summarize the morphological classes of BCM's, outline the structural meaning of these classes, particularly in differentiating the interior versus halo properties of BCM's, and demonstrate that the interior properties of BCM's are due to a history of mergers, while the extended envelopes are primordial in origin.

BCM's are extreme examples of a class of very homogeneous objects, elliptical galaxies. They are extreme in their luminosities (L > 10¹¹ L, H₀ = 100 km sec^-1 Mpc^-1 assumed throughout this review), being the brightest objects in the Universe that emit light strictly from stellar photospheres. They are extreme in their absolute sizes with some cD galaxies measuring over 1.0 Mpc in radius (Oemler 1976, Schombert 1984). They are also extreme in the environment in which they reside, the cores of rich clusters. In modern astrophysics it is generally perceived that special objects imply a special process of formation or evolution, and the process most often invoked is one of growth of BCM's from the cannibalism of lesser galaxies by dynamical friction. Note that there are certain dynamical reasons to believe this effect would not be strong in present-day clusters since, although the cross section for galaxy interactions are high in the cores of a rich cluster, those encounters are fast and not typical of the bound, merging orbits (see Merritt 1985).