1. Blitz, L., Fich, M. and Stark, A. A. 1980 in Interstellar Molecules, B. H. Andrew, ed. Reidel: Dordrecht p. 213.
2. Burton, W. B. 1971, Astron. Ap. 10, 76.
3. Burton, W. B. 1972, Astron. Ap. 19, 51.
4. Burton, W. B. and Gordon, M. A. 1978, Astron. Ap. 63, 7.
5. Cohen, R. S., Tomasevich, G. R. and Thaddeus, P 1979 in the Large Scale Characteristics of the Galaxy, W. B. Burton, ed. Reidel: Dordrecht p. 53.