One of the biggest surprises to emerge from the photographic
amplification process was the detection of faint shells around
otherwise apparently normal (but often isolated) elliptical galaxies.
The shells are now believed to be traces of ancient mergers. Many,
perhaps most ellipticals accumulate stars in this way, but the evidence
is soon smoothed out in the clusters of galaxoes where most elliptical
galaxies are found.
Among the first (and best) examples of a 'shell' galaxy was NGC 1344, shown here. This print is from the discovery paper. There were soon many other examples discovered and these were published as a catalogue. One of the more intriguing examples was NGC 3923.