If it wasn't for the hole in the shell,
Stomatella would be the most likely guess, but these little guys don't have a hole in the top of the shell. I'll guess that maybe you're seeing a
keyhole limpet, but a picture here certainly would help!
A good place to start is usually looking through the
Hitch-hikers FAQ and see if you can't find what you're looking for, or at least something that is at least a close approximation...
Rob