The ornamental design for a casket, as shown and described.
FIG. 1 is a plan view of a casket showing my new design.
FIG. 2 is a side elevational view thereof; and,
FIG. 3 is an end elevational view thereof.
Surface indicia in the nature of wood graining, shown in the drawing figures, is disclaimed as not being a part of the claimed ornamental design (MPEP §1504.01 A).