All expenses and disbursements in connection with the maintenance and operation of the office of supervisor of public utilities, including all salaries of clerks, assistants, engineers, accountants, and the duly appointed supervisor, shall be paid pro rata each month by the public service utilities (exclusive of those operating on an annual flat charge basis), which are subject to supervision by the supervisor of public utilities, under any law, charter provision or franchise requirement. The pro rata contribution of each public service utility shall be in relation to its preceding calendar year gross receipts and shall be a percentage in relation to the calendar year total gross receipts of all such public service utilities (exclusive of those operating on an annual flat charge basis); provided, however, that any direct pro rata contribution exempt by franchise provisions in which a per cent gross receipts tax is provided in lieu of direct contribution to the payment on the salary and expenses and charges of the supervisor of public utilities, and of his assistants and subordinates shall be paid by the city from its general fund and in conformity with required budgetary practice. The Dallas Railway & Terminal Company shall pay a pro rata contribution in the relation that its total gross receipts for the preceding calendar year bears to the calendar year total gross receipts of all such public service utilities (exclusive of those operating on an annual flat charge basis). The moneys collected under this section shall be deposited to the credit of the general fund of the city. (Ord. 6622)