Mo. Rev. Stat. § 50.1120

In the event a member has chosen an optional form of payment as provided in subsection 2 of section 50.1110 which provides for a continuing payment to a beneficiary after the death of the member in which the member received a reduced annuity during his or her lifetime and the member's beneficiary precedes the member in death, the member's benefit shall revert, effective the next month following the death of the member's beneficiary, to an amount equal to his or her normal annuity at the time of retirement plus any cost-of-living or other increases that the member may have received prior to the member's beneficiary's death.