The ornamental design for a medallion, as shown and described.
FIG. 1 is a front perspective view of a medallion, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left-side elevational view thereof, the right-side elevational view being a mirror image thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines showing a Roman numeral in the drawings depict environmental matter, and form no part of the claimed design. The rear elevational view forms no part of the claimed design.