The ornamental design for a shelf, as shown and described.
FIG. 1 is a fragmentary isometric view of a shelf, according to our new design;
FIG. 2 is an isometric view;
FIG. 3 is a right-hand side elevational view;
FIG. 4 is a left-hand side elevational view;
FIG. 5 is a front elevational view;
FIG. 6 is a rear elevational view;
FIG. 7 is a top plan view; and,
FIG. 8 is a bottom plan view.
The end of the shelf that is not shown in FIGS. 1 and 2 is a mirror image of the end that is shown in FIGS. 1 and 2. The pairs of spaced apart broken wavy lines extending across the center portion of the shaft are break lines. The appearance of any portion of the shelf between the break lines forms no part of the claimed design. In FIGS. 1, 2, 7 and 8, the straight dot-dash lines extending parallel to the front edge of the shelf represent unclaimed boundaries that form no part of the claimed design. In FIGS. 3 and 4, the broken vertical lines at the ends of the upper and lower solid lines represent unclaimed boundaries that form no part of the claimed design. Except as specified above, the dashed lines in the drawings show environment that forms no part of the claimed invention.