As a precursor to the luminosity classification of all galaxies, van den Bergh's [190] division of the intrinsically lowest luminosity galaxies into four groups is outlined in the table below. The so-called ``nucleated dwarfs'' described in [148] must be added to this classification; the prototype of this sub-class is IC 3475 (see the table below). These specific low-surface-brightness objects have luminous knots but no global structure other than nuclei in a number of instances. In fact about thirty percent of the dwarfs in the Virgo cluster have discernible nuclei, unlike the Local Group in which none of the dwarfs possess nuclei [149]. The nucleated dwarfs in Virgo are however 3-5 mag brighter than the dwarf spheroidals in the Local Group.

Photographic images of 138 dwarf galaxies in the Virgo Cluster have been examined [158a], following [150], and have been systematized into the various types of morphology encountered there, noting in the first instance that there are no real dwarf spirals in the sample. The following types of low-luminosity galaxies were found: (a) Dwarf Ellipticals dE are the most common type of dwarf in the Virgo Cluster. They are characterized by a smooth intensity distribution over the face of the galaxy, and by their low surface brightness which is ``an unfailing indicator of low intrinsic luminosity''. Some members of this class have a central star-like nucleus. (b) Dwarf S0 galaxies dS0 are still only tentatively identified as an entirely new class of objects. They are characterized by the presence of two components, a disk and a bulge, both of which are smooth and symmetric. The surface brightness of dS0 galaxies is also low, as with the dwarf ellipticals. (c) Sm galaxies still show some fragmentary evidence of spiral structure, while Im galaxies are totally chaotic. (d) Huge Im and dE Types are very low surface brightness galaxies with large (up to 10 kpc) diameters. (e) Blue Compact Dwarfs, BCD galaxies consist of several knots and show some low surface brightness fuzz of an underlying structure. Finally, (f) Galaxies with BCD components. These objects are thought to be basically Im galaxies with a small number of intense star-formation events dominating their presently observed morphology.

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