/etc/X11/xorg.conf.d/50-marblemouse.conf

     Section "InputClass"
     Identifier "Marble Mouse"
    # MatchProduct "Logitech USB Trackball"
    # MatchIsPointer "on"
    # MatchDevicePath "/dev/input/event6"
    # Driver "evdev"
     Option "Emulate3Buttons" "true"
     Option "EmulateWheelButton" "8"
     Option "EmulateWheel" "true"
     Option "XAxisMapping" "6 7"
     Option "ZAxisMapping" "4 5"
     Option "ButtonMapping" "1 2 3 4 5 6 7 8 9"
    EndSection


    Section "InputClass"
    Identifier      "Marble Mouse"
    MatchProduct    "Logitech USB Trackball"
    MatchIsPointer  "on"
    MatchDevicePath "/dev/input/event*"
    Driver          "evdev"
    Option          "SendCoreEvents" "true"

    #  Physical buttons come from the mouse as:
    #     Big:   1 3
    #     Small: 8 9
    #
    # This makes left small button (8) into the middle, and puts
    #  scrolling on the right small button (9).
    #
    Option "Buttons"            "9"
    Option "ButtonMapping"      "1 8 3 4 5 6 7 2 9"
    Option "EmulateWheel"       "true"
    Option "EmulateWheelButton" "9"

EndSection