Upon the death of a member prior to retirement who was a member continuously for not less than 18 months immediately prior to his death, survived by an eligible spouse, if such eligible spouse waives the amount, if any, payable as a death benefit pursuant to Section 31781, such eligible spouse shall be paid a monthly survivor’s allowance equal to the sum, based upon the member’s compensation at the time of his death and the number of children for which such eligible spouse is caring, specified in Section 31849 minus the amount to which such eligible spouse is entitled pursuant to Section 31765, 31765.1, 31781.1, 31781.2, or 31787.