Rossel Island, about 200 miles southeast of New Guinea, can lay claim to one of the most novel and complicated primitive monetary systems, one in which time was a significant factor in measuring the value of goods. The actual pieces of money had been handed down virtually unchanged to successive generations since time immemorial and allegedly were of divine origin.
Rossel money split into two variations. Dap money came in single polished pieces of shells, and ko money in
sets of 10 discs made from shells. Dap money covered a larger range of values and stretched into the smallest values, whereas ko money exchanged hands in the larger transactions. These two variations bore some gender connotation, dap money was looked on as men’s money, and ko money as women’s money. Some goods were only priced in one type of money, and other goods in a combination of dap and ko money.
The system of denominations of shells of different values made the Rossel money unique among primitive
currencies. The actual names were a bit clumsy, but 22 different values are represented. For simplification it
is easiest to regard the lowest denomination as number 1, the next lowest denomination as number 2, and so on, until the largest denomination of number 22 is reached. Dap came in all 22 denominations, but ko came only in
denominations of numbers 8 through 22. One denomination was not a multiple of other denominations, and no one denomination was equivalent to a fixed number of other denominations, contrary to the U.S. monetary system in which 100 pennies equal a dollar. A good costing a number 10 could not be purchased with anything but a number 10, and not in a multiple of smaller denominations.
The differences in value between denominations were based on the amount of time one denomination would have to be loaned out before repayment could be required in another denomination. If a number 10 was loaned out for a length of time, the loan had to be repaid in a number 11. A loan of a number 10 for longer
lengths of time called for repayment in a number 12, or a higher denomination, depending on the length of the loan. 
Transactions involved a highly elaborate system of credit. Suppose individual A sought to purchase a product
priced at number 10 in dap, and this individual owned denominations below number 10 and above number 10, but not in number 10. This individual would borrow a number 10, and would repay the loan in the future with a denomination higher than a number 10, as a means of paying interest. This individual would not mind this arrangement, having the opportunity to loan out his own dap denominations and earning interest also. If a number 22 was loaned out, an initial interest payment was made in a smaller denomination, and in the future the loan was repaid with another number 22. A special class of brokers arranged these necessary transactions.
The currency units, or shells, of the higher-valued denominations (number 18 and above) were known on an individual basis by active financial traders. Only seven currency units of denomination number 22 were in existence, all owned by chiefs. The higher values were considered sacred. When a number 18 exchanged hands, the parties involved crouched down. Numbers 19 to 22 were kept enclosed, always protected from the light of day.
See also: Yap Money 
Armstrong, W. E. “Rossel Island Money: A Unique Monetary System.” The Economic Journal 34 (1924): 424–429.
Einzig, Paul. 1966. Primitive Money.