4.1. Extensions of the Sequence Beyond Sc
Galaxies originally classed Sc on the Hubble system cover a large interval along the sequence, ranging from regular well-developed arms in the early Sc to nearly chaotic structures in the very late Sc. de Vaucouleurs has made a division and extension of the Sc and SBc families by introducing the cd, d, dm, m, and Im subdivisions. This important addition extends the Sd class of Shapley and Paraskevopoulos (1940) and Shapley (1950) toward even later types. The Sd type-example is NGC 7793 (photograph in Shapley 1961, p. 22), which would have been classed as a very late Sc on the Hubble system.
The further extension beyond Sd by inclusion of Magellanic Cloud types (Sm or SBm, or SAm and SBm in the revised notation) follows from the discovery of weak but definite spiral structure in the Large Magellanic Cloud (LMC) (de Vaucouleurs 1954, 1955a). Closely related galaxies, collectively denoted Irr by Hubble (as were the Sm types) but clearly following linearly beyond the Sm stages, are the dwarf Population I systems such as IC 1613, NGC 2366, Holmberg I and II, and IC 2574 (Atlas, pp. 39, 40). de Vaucouleurs's symbols I(m) and IB(m) replace Hubble's Irr class and show the connection with the Magellanic Cloud types. The revision brings the type back into the formal sequence as suggested by Lundmark (1927).