Shell money most valued by the Native American tribes of the Pacific Coast from Alaska to northwest California was "Dentalium shell", a species of long narrow marine shelled mollusk, a tusk shell or scaphopod. The tusk shell is naturally open at both ends, and can easily be strung on a thread. This shell money was valued by its length rather than the exact number of shells; the "ligua", the highest denomination in their currency, was a length of about 6 inches. Farther south, in central California and southern California, the shell of the olive snail "Olivella biplicata" was used to make beads
Shell money on the back or upper side of the shell, was commonly used. Many specimens were found by Sir Austen Henry Layard in his excavations at Nimrud in 1845–1851. Shell money Shell money is a medium of exchange similar to coin money and other forms of commodity money, and was once commonly used in many parts of the world. Shell money usually consisted either of whole sea shells or pieces of them, which were often worked into beads or were otherwise artificially shaped. The use of shells in trade began as direct commodity exchange, the shells having value as body ornamentation.