Types of Shell
An expanded shell is a coin shell that fits over a regular coin. Many years ago, you would need to buy a shell and a milled down down to fit it, but nowadays an expanded shell is the standard shell for most magic tricks.
Super Expanded Shells
The "super" expanded shell is meant to fit over a typical expanded shell. In this way you can nest three coins in what appears to be just one coin.
Slippery Sam Shells
A Slippery Sam Shell is often used for routines such as Three-Fly. In this gimmick only half of the edge of the coin is remaining, allowing you to slide the coin into the shell and have it "lock" in place only when fully nested. This type of gimmick was popularized by Steve Dusheck and invented by Edward Bagshawe in the late 1920s.
Shim Shells are expanded shells that have a thin piece of metal hidden inside that allow them to be attracted by a magnet. These can then be used in conjuction with gimmicks such as a Raven to steal away the shell.
Repairing a shell
Stuck shell? Or need to expand it a little more? Read our blog post to learn how to repair an exapnded shell.
It seems that Penn & Teller's use of the word "turtle" to to refer to a coin shell on their Fool Us television show has moved into general magic terminology and many newcomers to magic now use that word in place of a shell.