Originally Posted by
AlphaDogg
It's called a claw derailleur
+1. You can get adapters that allow you to attach a conventional rear derailleur to them and then attach to the bike as the claw does.
Look here:
http://sheldonbrown.com/harris/derai...ar.html#hanger
They have an "Adapter Claw" available for $4. Also, Harris offers a Tourney Mega Range rd with the claw mount shown above on this page in the chart of the derailleurs they sell.