The board of trustees annually shall allow regular interest on the mean amount for the preceding year in each of the funds with the exception of the expense fund. The amount so allowed shall be due and payable to said funds and shall be annually credited thereto by the board of trustees from interest and other earnings on the moneys and other assets of the retirement system. Any additional amount required to meet the interest on the funds of the retirement system shall be paid by the said cities and any excess of earnings over such amount required shall be deductible from the amount to be contributed by the said cities.