Any contributor who has reached superannuation retirement age may retire for superannuation by filing with the board a written statement duly signed by the contributor setting forth at which time he desires to be retired. Said application shall retire said contributor at the time so specified.

On retirement for superannuation, a beneficiary shall receive a retirement allowance which shall consist of (1) a member's annuity which shall be the actuarial equivalent of his accumulated deductions standing to his credit in the members' annuity reserve account, and (2) a city annuity equal to one one-hundred-twentieth of his final salary multiplied by each year of total service and in addition thereto in the case of an original member one one-hundred-twentieth of his final salary multiplied by each year of prior service. At any time, the board, by rule, may, by increasing the city annuity, authorize the payment of a minimum retirement allowance of one hundred dollars ($100) per month to every beneficiary who has retired for superannuation after twenty years' service, but the total city annuity shall never exceed fifty per centum of the final salary.

Whenever the board has, by rule, authorized the members of the retirement system to transfer from the one one-hundred-twentieth class to the one one-hundredth class, or whenever the board shall hereafter, by rule, authorize the transfer to the one-eightieth class, the city annuity shall be calculated as follows: