panel 9 down toward the tonneau configuration conversely pivoting the side panels 7 upward and outward causes the roof panel 9 to be raised toward the shell configuration the components of the centering linkage assembly 35 cooperate to retain the roof panel 9 centered over the pickup bed 18 during conversion of the cover mechanism 1 between its shell and tonneau configurations fig7 illustrates the cover mechanism 1 at a stage of conversion just below the fully raised shell configuration fig8 illustrates the mechanism 1 at a stage of conversion just above the fully lowered tonneau configuration referring to fig9 the rear hatch assembly 11 includes a rear hatch frame 45 which is pivotally connected to the roof panel 9 as by internal hinge members 47 which enable the frame 45 to pivot inwardly toward the front of the pickup truck 19 hatch cylinders 49 such as hydraulic cylinders are pivotally connected between the rear hatch frame 45 and the roof panel 9 and enable the frame 45 to be pivoted between an extended vertical position above a tailgate 51 of the truck 19 in the shell configuration of the cover mechanism 1 and a retracted horizontal position beneath the roof panel 9 in the tonneau configuration preferably the hatch cylinders 49 and side panel cylinders 27 are coordinated in such a manner that the rear hatch frame 45 is raised to its horizontal position prior to retraction of the side panels 7.