Interesting thought on melodiums/ia??, Brian. I was trying to discuss modes earlier on the thread.

Maybe your idea only works, as you will be aware, if we exclude the ionian (ie major scale) from our category of modes.

Melodeons, for those who don't know, are like some mouth organs, but instead of 'blow' you have 'push' and instead of 'such' you have pull. Similar pattern of push/pull over the main octave as for a single key mouth organ. I assume this is the type of instrument Walter Pardon had. In this picture he appears to be playing a two row diatonic Hohner Pokerwork model.

http://www.mustrad.org.uk/articles/pardon2.htm

Like diatonic mouth organs, the melodeon can get the notes of the relative minor key, the 'natural minor' ie the Aeolian mode. This would end with extended bellows/ suck on harmonica.

And what else?

I'm guessing it must be possible to play 'cross melodeon' in the same way that people play 'cross harp', and, I am hazy about all this, but I think it might bring in the mixolydian. So on a C instrument you have the notes for G mixolodian (G A B A D E F G), but you would end on a push ie with bellows close together. Playing in G on a C instrument is 'cross harp'. But you would end on a push, ie bellows not in extended position.

You could work this out for all the 'church' modes through the melodeon diatonic scale; some end push some pull.

However, according to Roud/Bishop, many 'folk songs' are in ionian. So it looks as if Walter Pardon and Roud/Bishop disagree here.

And I haven't even thought about pentatonic melodies, and their varieties.

Complicated????