Limpet.

Here is a limpet shell. Have you ever heard the phrase, "cling like a limpet"? That's what limpets do. They cling onto other shells for transportation and shelter. Notice how this one is curved. It probably lived in some whelk or moon snail shell as a very small limpet, and grew larger and larger, and curved as the shell was curved, to accommodate its tight lodging. And yes, my fingernails need trimming.