There shall be a treasurer of the board of trustees, who shall be appointed by it and subject to its rules and regulations. He shall be the treasurer of the assets of the retirement system. Payments of the funds of the retirement system shall be made by the treasurer only upon vouchers signed by the treasurer and countersigned by one other person designated by the board of trustees. The treasurer of the board of trustees shall furnish to the board of trustees a surety bond of a company authorized to do business in this state in such an amount as shall be required by the board of trustees, provided that if the treasurer is a corporate trustee authorized to do business as such under the laws of this state, the trustees, in their discretion, may decide that no such bond be furnished. The premium on the surety bond shall be paid from the expense fund.