Holmberg  first divided irregular galaxies into two types: Type I Irregulars are highly resolved systems similar to the Magellanic Clouds, while Type II Irregulars show a smoothed ``amorphous'' distribution of light, often broken by irregular patches of obscuration. Type I Irregulars have subsequently found a natural home as Im galaxies following the late spirals of class Sd and Sm [27 ]. Their star-formation history is summarized in ; while global radio, optical and spectral properties are reviewed in [42, 55]. On the other hand, Sandage and Brucato  have suggested that the original Irr II galaxies can be sub-divided, and that a single class should be abandoned. The properties of the Irr II class galaxies (also called I0 in ) have been summarized in . Those few objects which show no spiral structure, but have unresolved disks, are now termed amorphous galaxies . Examples are NGC 3077, NGC 5253, and M82.
|Morphological Type||(Hubble Revised)||Code t|
|Compact (high density) Ellipticals||cE||-6|
|Dwarf (low density) Ellipticals||dE||-5|
|Normal Elliptical Systems||E||-7|
|Giant Ellipticals with extended optical coronae |
(in particular the Morgan type cD)
|Irregular Systems of Type II||I0|
|Spiral Systems||Sa |
|Spirals of Magellanic Type||Sm||9|
|Irregular of Magellanic Type (=Irr of Type I)||Im||10|
|Compact Blue Irregulars (Isolated Extragalactic |
H II Regions)
Heidmann et al.  (above table) introduced a numerical code of the morphological types in the revised Hubble system. This code was extended and used in the Second Reference Catalogue of Bright Galaxies, RC2 ( in Secondary Galaxy Catalogs).
More detailed tables for coding of Hubble types, revised morphological types (de Vaucouleurs ), DDO (van den Bergh) types and Yerkes (Morgan) types etc. are given in the RC2.