1. V. A. Ambartsumian, The Structure and Evolution of Galaxies, 13th Conference on Physics, Univ. Brussels (1965).
2. H. L. Arp, Sky and Telescope,38, 385 (1969).
3. H. L. Arp, Astron. Astrophys.,3, 418 (1969).
4. P. C. van der Kruit, J. H. Oort, and D. S. Mathewson, Astron. Astrophys.,21, 169 (1972).
5. J. H. Oort, M. N.,32, 15 (1973).