The shell most valued by the Native American tribes of the Pacific Coast from Alaska to northwest California was Dentalium, one of several species of 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.[citation needed]