The Arrangement of Dante's Purgatorial Reliefs

The description of the marble reliefs which depict examples of humility, in the tenth canto of Purgatory ,—the Annunciation, David and the Ark, Trajan and the widow—has been universally admired, and often studied and discussed. But it seems to me, for several reasons, that the specific matter of the order in which their details are presented may certainly be considered further with interest, and perhaps with profit—although the fact of their orderly sequence is, in at least a general way, obvious to every reader.