Arp [1] first drew attention to ellipticals with shells. These were further illustrated in Arp and Madore [2] and catalogued by [195, 216]. A review of their status is given in [61], and also [217] which outlines a three-way classification scheme: Type 1 include systems whose shells are aligned; Type 2 have position angles of the shells randomly distributed around the central galaxy; Type 3 includes all other systems where there are only a small number of shells, or where the shells are not manifest as well-defined concentric structures.

Additional discussion on Shell Galaxies can be found in:

1. Carter, D., Prieur, J.L., Wilkinson, A., Sparks, W.B., & Malin, D.F., Mon. Not. R. Astron. Soc. 813, 235 (1988)
2. Malin, D.F., & Carter, D., Astrophys. J. 534, 274 (1983)
3. Schweizer, F., & Seitzer, P., Astrophys. J. 88, 328 (1988)
4. Taniguchi, Y., Ichikawa, S., Hamabe, M., Yamagata, T., & Iye, M., Astron. Astrophys. 385, 233 (1990)
5. Wilkinson, A., Browne, I.W.A., & Wolstencroft, R.D., Mon. Not. R. Astron. Soc. 933, 228 (1987)