The design consists of the features of shape and configuration applied to a MATTRESS as illustrated in solid lines in the attached drawings, the stippled lines illustrating features of the article which form no part of the design.Drawings of the design are included wherein:FIGURE 1 is a front elevational view of the article to which the design is applied, namely, a mattress;FIGURE 2 is a rear elevational view thereof;FIGURE 3 is a left side elevational view thereof;FIGURE 4 is a right side elevational view thereof;FIGURE 5 is a top view thereof;FIGURE 6 is a bottom view thereof; andFIGURE 7 is a perspective view thereof.