The ornamental design for shell, as shown and described.
FIG. 1 is a front perspective view of shell showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back perspective view thereof;
FIG. 4 is a left side view thereof, the right side view being a mirror image thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a top view thereof; and
FIG. 7 is a bottom view thereof.
The broken lines, in part, illustrate potions of the shell shown for context and do not form any portion of the claimed design.
The shading lines shown in the drawings represent an approximate three-dimensional contour of the design, and are not intended to indicate surface decoration.