Window Manager and Login Screen Removed After Trying and Uninstalling Gnome Shell

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http://askubuntu.com – A while back I decided to try the Gnome Shell desktop environment as I've always been a fan of the foundation and the desktop environment. I decided that I'd become used to Unity and that I wouldn't really log into a GNOME session often, so I uninstalled it via the command line... which, frankly, was stupid. When I boot into my Ubuntu installation, the login screen does not appear, but rather a logging TTY - the one seen upon shutdown that is reserved for X11, and normally accessible by Ctrl+Alt+F7. Can anyone tell me the packages required to make the login screen appear - as in default packag (HowTos)