Assessment as firm: specified partnership instrument and certified deed required; failure disallows partner remuneration and interest. Assessment as a firm requires the partnership to be evidenced by an instrument with the individual shares of partners specified. A certified copy of the partnership instrument must be furnished with the return when assessment as a firm is first sought; revised certified instruments are required after constitution changes. Failure to comply leads to full disallowance of interest and remuneration in the hands of the firm and non taxability of those amounts in the hands of partners.