References for those who are interested
L. Allen Smith, "Toward a Reconstruction of the Development of the Appalachian Dulcimer", Journal of American Folklore, Vol 93, No 370, pp. 385-396 (1980)
L. Allen Smith, A Catalogue of Pre-Revival Appalachian Dulcimers, University of Missouri Press, Columbia, 1983 (Foreword by Jean Ritchie).
The Tyrolean scheitholt, the French epinette des Vosges, the Dutch Hummel and so forth, seem to me to be, from a performer's point of view, largely the same instrument. This is why I think that the scholars who try to establish that the ancestor of the mountain dulcimer was, say, the scheitholt and not the epinette des Vosges will may never reach a clear answer. The appalachian dulcimer might have been developed from any than one of the European antecedents. It is even possible to imagine circumstances in which more than one of the European fretted zithers contributed to the evolution of the Appalachian dulcimer.
I consider the hammered dulcimer to be a species of psaltery, rather than a species of zither. But even I realize that that is just hairsplitting.
The mountain dulcimer is perhaps more popular today than anytime in its 450-year history (reckoning from the 1560 date given by the New Grove first edition for a fresco in Rynkeby, Denmark, depicting a fretted zither). Folks are experimenting with numerous techniques and tunings. It's an exciting time to be a dulcimer player.