As three of you immediately identified in your comments, last week’s crocodile mystery was the fastening in the center of a volvelle, holding the various layers in place and allowing them to turn: Volvelles are paper wheels that are fastened to a leaf so that the discs spin independently. Some of the earliest volvelles were used for prognostication; Ramon Llull is credited with bringing the volvelle to the West in the late thirteenth century for use in his Ars Magna.  … Continue Reading