Jamey has correctly pointed out to me in a private e-mail that "wampum" beads were only made by Native Americans in the NE from two different shell types. Some of the wampum beads made their way to the NW through trade or other means, as I have a number of examples found in the NW.
Upon re-examination, the white beads in the Chief Tom Yallup necklace are graduated clamshell beads.
I am sorry for any confusion this posting may have generated.
Thank you Jamey for pointing out my error.