By Kenneth Maddock* is a custom prescribing that a man make an armed assault upon his sister should he hear her sworn at obscenely. The earliest account of the custom is given by Warner for the Murngin. 1 The only other first-hand account is by Hiatt for the Burera. 2 During field work on the Beswick Reserve, 8 I found a like custom among the Dalabon and neighbouring tribes whose now abandoned territories are in the south and centre of the Arnhem Land Reserve. In the present paper I shall define the mirriti, summarize my field data, consider explanations suggested by Warner, Hiatt and Makarius, 4 and propose a structural interpretation. A preliminary question concerns the distribution of the mirriri. The Beswick Reserve is inland about 190 miles from Maningrida, where Hiatt worked, and 210 miles from Milingimbi, where Warner worked. Maningrida and Milingimbi are about 45 miles apart on the north coast of the Arnhem Land Reserve. The apparent wide distribution of the mirriri is misleading. Before modern population movements, the tribes practising the custom were much closer to one another. Warner's map of the Murngin area puts a tribe, the Boun, to the south-west. 5 The Dalabon or Ngalgbon, names not reported by Warner, are also known as Buan, and presumably are his Boun. The map puts them inland about 75 miles from the Burera and 45 from the Murngin proper. The Burera and Murngin proper are about 45 miles apart on the coast. Warner gives little information about the Boun. Unlike the Burera, they are not classified by him with the Murngin. Since Warner's field work (1926-29), the interior of the Arnhem Land Reserve has virtually emptied, the Aborigines migrating north and east to mission and government settlements alone: the coast or south and west to