What is on these hermit shells???

So I got some new hermits in and they have some stuff on their shells. I can't figure out what they are. They almost look like little shells on top of the shells. I thought I saw little things sticking out from underneath but might of been my paranoia. So can anyone ID these things? Are they pests?

 

Slipper Limpets, they bore through snails shells, I believe they are parasitic, really not a concern on the Hermits shells nor will they spread. They are after the snails that once lived in those shells.

 

Awesome Thanks