Mercury 4 stroke lower unit problems

[jftyler1965]

I have a Mercury 4 stroke, 50 HP, bigfoot engine. I replaced the water pump impeller myself and ran into a small issue. It is concerning the rubber grommet or "face seal" that slides down the shaft and seals the pump housing and the drive shaft. This is shaped like a "mushroom". Does the bottom half of this grommet go down into the pump housing, or does the whole grommet sit on top of the pump housing? I did not pay enough attention when I took off. Thanks for any advice.