QUOTE(boogiecat @ Oct 18 2007, 09:15 AM)
Well, yes, I had thought that it might mean tremolo. There is also an ornament written above though, and I can't get my head round how it is physically possible to tremolo on about 10 notes and play the ornament all in the space of a crotchet.
I don't have the AB edition, I'm referring to a freebie Internet edition (from sheetmusicarchive.net if you want to see my frame of reference) but in those bars, you have to use the thumb to play two notes - for instance, in bar 25, the thumb would play the lower g and a. That leaves you fingers 4 and 5 free to play the ornament.
Ornaments are only ever played on the note by which they are written. If a piece requires a double ornament, there will be two ornament signs in place - one by each note to be ornamented. So in this case, only ornament the top note (with fingers 4 and 5 as discussed above) whilst holding the other notes down.
And finally, if you don't actually know what ornament to play, it seems an up mordent would be a reasonable choice.