Halton Arp's "Atlas of Peculiar Galaxies" was meant to provide examples of various types of galaxies as a guide for study of the origin and evolution of galaxies, at a time (the mid-1960's) when astronomers had little idea what that involved. Although the Atlas was intended only as a sampling, rather than an inclusive listing of galaxies, it contains a number of interesting objects, which are often referred to by their Arp numbers. The table below is based on Arp's description of each "group" of catalog entries. For each entry, the NGC / IC / PGC identification is shown; a plus sign indicates a multiple galaxy with more than one catalog number.