One method used to collect clams involves a series of hooks that when lowered to the river floor would allow the clam to attach itself. After a period of time the harvester would pull the assembly up and detach the clam.
Once the meat was gathered the shell was sold to the button factories along the river. Using mechanical means the shells were cleaned, drilled for the button blank, cut, polished and packaged. The pearl button business declined in the 1960s with the availability of a cheaper to make material - plastic.
Drilled shells like the one above now in the at the National Mississippi River Museum in Dubuque, Iowa can still be found today where button factories once stood. Try looking in Guttenburg, Iowa for button shells. Know that.