The taxes levied under this article shall be payable in quarterly installments on or before the expiration of thirty days from the end of the quarter in which they accrue. The taxpayer shall, within thirty days from the expiration of each quarter, make out a return upon a form prescribed by the City Clerk, showing the gross proceeds of sales, or gross income of business, trade or calling and compute the amount of tax for which he is liable for such month, verify it by oath, and mail it, together with a remittance in the form required by Section 767.19 for the amount of the tax to the office of the City Clerk. The Clerk, if he deems it necessary to insure payment of the tax, may require return and payment under this section for other than quarterly periods. If for any reason it is not practicable for the individual taxpayer to make the oath, the oath may be made by any duly authorized agent.