The use of the cowry currency gradually spread inland in Africa. By about 1850 the German explorer Heinrich Barth found it fairly widespread in Kano, Kuka, Gando, and even Timbuktu. Barth relates that in Muniyoma, one of the ancient divisions of Bornu, the king's revenue was estimated at 30,000,000 shells, with every adult male being required to pay annually 1,000 shells for himself, 1,000 for every pack-ox, and 2,000 for every slave in his possession.