General: What's this peice called??

I have a geared bike with a single chainring up front. I'm going to put a chain guard in place of the large ring, and I would like to get one of those little plastic pieces that clamps around your seat tube and keeps the chain from falling off on the inside. The only problem is I don't really know what the name of that part is, so I'm having trouble tracking one down. if anyone that could point me in the right direction, I'd appreciate it.

Third Eye Chain Watcher

Thanks guys, I stopped in a shop today and described it to a guy, and he had one of the 3rd eye parts so I grabbed it. I unfortunately don't know if this thread can be deleted, so it'll just have to sit here taking up space.