10mm discs with ruffled edges

[Robin]

I have tried to research these 10-11mm discs that were strung onto a necklace. They are softer than most seashells, being porous similar to eggshell. They are fragile, you can see many have broken. The ruffles are on both sides, but one side has pronounced texture and the other side has gentler undulations. They vary greatly, each unique.

Someone suggested they might be forminifera, most of which are benthic organisms. I found some disc shaped varieties called nummulites. I did not find this variety.

Help!! What are they??