The selection of N. gibbosulus in Mediterranean web sites and N. kraussianus is South African web pages is really a purpose of their availability in the vicinity in the web pages, but doesn't make clear why this genus was most popular more than other shells of similar dimensions and shape. https://medium.com/@spoilmevintagediy/the-rich-history-of-beadwork-a-journey-through-the-ages-4e4fd2e474cf