1. Arp, H., Atlas of Peculiar Galaxies (California Institute of Technology, Pasadena, 1966).
2. De Vaucouleurs, G., and De Vaucouleurs, A., Reference Catalog of Bright Galaxies (Univ. Texas Press, 1964).
3. Arp, H., Science, 174, 1189 (1972).
4. Allen, R. J., Astron. Astrophys., 7, 330 (1971).
5. Spitzer, L., Diffuse Matter in Space (Interscience, New York, 1968).