1. Arp, H. C.: 1966, Astrophys. J. Suppl. Ser.
14, 1.
2. Babcock, H. W.: 1939, Lick Observ. Bull.
19, 41.
3. Burbidge, E. M. and Burbidge, G. R.: 1964a, Astrophys. J.
140, 1445.
4. Burbidge, E. M. and Burbidge, G. R.: 1964b, Astrophys. J.
140, 1617.
5. Burbidge, E. M. and Burbidge, G. R.: 1970, in Stars and Stellar Systems
9, Univ. of Chicago Press, Chicago, in press.