spiral structure is rather more open. In Sc nebulae the nucleus is very small, becoming quite inconspicuous in type Sc+. The spiral arms are very loosely wound and have a patchy structure due to the presence of supergiant stars and emission nebulae. It will be noted that the classification scheme presented above does not distinguish between normal spirals and barred spirals. According to Holmberg,1 the irregular galaxies may be subdivided into two types. Irregular galaxies of type I are blue and contain supergiant stars, while those of type II are red, and might more appropriately be called pathological galaxies. An example of the latter type is NGC 3034 = M 82.