Mail 6.0?

I just upgraded to Mountain Lion.When I had Lion, I opened my mail, clicked on first message,read it, then deleted it, and arrowed down to next message, it opened in the same window, was able to read all of my messages in one window. I have no idea what I changed, but now, I need to click on every single message I have one by one. I have absolutely no clue what has changed. It sucks having to open each e-main one by one. Any ide how I can change it back to what it used to be? Thanks alot for the help.