Key points are not available for this paper at this time.
The supernova frequency in Shapley-Ames galaxies of types Sc-Sd shows a sharp peak in objects with inclinations i <~ 25ᵈeg^. This suggests that many supernovae occur at the bottoms of deep steep-walled pits in the equatorial layer of gas and dust in late-type galaxies. Such pits might have been excavated by previous supernovae in rich clusters or associations.
Bergh et al. (Wed,) studied this question.