x-5 xdrive 35d air filter

[vruss7]

I was checking the air filter on my X-5 xdrive35d and disconnected the duct on the left side by unscrewing two screws to pull it apart. In doing so a large rubber gasket fell out - (I won’t go into the details, but be careful not to let this o ring fall down into the engine). My questions is what is the proper placement to reinstall this o ring 1)put it on the male part of the duct or 2) seat it into the female part. I got the duct reconnected but not sure if the o ring seated properly. Any advice appreciated...