Can anyone tell me how to get this faucet off my basin?

This is the top of the sink

http://img517.imageshack.us/img517/9...ovesinkoj1.jpg

This is the underside

http://img443.imageshack.us/img443/7...dersinkrx2.jpg

I tried turning the screwlike thing with a screwdriver, but that got
me no where. Am I supposed to rotate the 1/2 moon somehow or is there
another trick, like taking a part off the top of the faucet first?

Thanks much