Instrument of payment and a symbol of wealth and power, the cowry shell was the common currency of West Central Africa well into the 19th century.

In Western Africa, shell money was common currency. Before the abolition of the slave trade, large shipments of cowry shells were sent to some of the English ports for reshipment to the slave coast. It was also common in West Central Africa as the currency of the Kingdom of Kongo called locally Nzimbu.

Shell money usually consisted either of whole sea shells or pieces of them, which was usually worked into beads or artificial shapes. The use of shells in trade began as direct commodity exchange, as the shells had value as body ornamentation.

There have been some forms of shell money found on just about every continent. However, the most used form was the shell of Cypraea moneta, the money cowry. At the time, trade money was highly important in Africa, South Asia, and East Asia.

The value of the cowry was much greater in West Africa than in the regions from which the supply was obtained. The trade was extremely lucrative, and in some cases, the gains are said to have been 500 percent. In the countries on the coast, the shells were fastened together in strings of 40 or 100 each, so that fifty or twenty strings represented a dollar. However, in the interior, they were laboriously counted one by one, or, if the trader were an expert, five by five. The monetary currency of Africa eventually gave way to modern currency.

source:

http://faculty.las.illinois.edu/m-saul/documents/SaulMoneyinColonialAA.pdf