Those look like trochus. Not a preferred shell for purple pinchers, but often found on small crabs in the wild. Here's what the shell guide had to say about them. See if the picture matches your shells:
http://www.hermitcrabassociation.com/ph ... 24&t=92552
Excerpt below:
8.) Tectus conus, or any shell of the genus Trochus a.k.a. strawberry top, cone shell
These shells are very attractive and look somewhat like turbo shells. They often come in green and red, but other colors do exist. Some hermit crabs in the wild (especially the young ones) will sometimes wear these shells. But in captivity, they prefer other types, and will rarely go back to this genus if given the choice. The spiral goes up instead of to the side, and sticks out quite far. The shells are also very thick, heavy, the openings are small, and there are often ridges in the opening. The crabs will spend a lot of extra energy carrying this shell, only small ones will fit in the opening, and the ridges will prevent most crabs from changing into them.