Judgment :- For the assessment year 1966-67, the Deputy Commercial Tax Officer, Erode (Rural), determined the total and taxable turnovers of the petitioner at Rs. 32, 65, 157.57 and Rs. 7, 79, 293.69 respectively. The petitioner took up the matter in appeal before the Appellate Assistant Commissioner, Commercial Taxes, Erode, challenging certain additions and effectually complaining against the original order of assessment. The Appellate Assistant Commissioner passed an order on 15th December, 1967 determining the total and taxable turnovers at Rs. 32, 06, 520.39 and Rs. 7, 14, 144.43 respectively. The case of the petitioner is that the said taxable turnover, as is seen from the appellate order, includes the purchase turnover of raw hides and skins effected locally amounting to Rs. 2, 80, 149.12. His case is that during the year in question the raw hides and skins so purchased by him, and which was subject to tax in accordance with the provisions of the Tamil Nadu General Sales Tax Act, were dressed and sold during the year in question and all such sales of dressed skins effected, but from and out of the pool of raw hides and skins brought to tax under the impugned order, are to be taxed differently under item 7(b) of the Second Schedule to the Act. It is common ground that this contention was not raised by the petitioner before the Appellate Assistant Commissioner. Therefore, the appellate authority concluded the appeal by including the entire purchase turnover of raw hides and skins in the skins in the assessable turnover and brought them to tax under item 7(a) of the Second Schedule to the Act. The petitioner, after coming to know of a decision of this court in Sadak Thamby & Co. v. Appellate Assistant Commissioner of Commercial Taxes on which ratio apparently he based his further representations for rectification of the appellate order, filed before the appellate authority an application for rectification on 3rd October, 1969. On 23rd October, 1969, the Appellate Assistant Commissioner, Salem, returned the representation with the following endorsement. "The appellate order was passed on 15th December, 1967, that is, long before the judgment referred to above, was pronounced. Hence the question of revision under section 55 does not arise." * It is an against this order, the present writ petitions are filed. On 23rd October, 1969, the Appellate Assistant Commissioner, Salem, returned the representation with the following endorsement. "The appellate order was passed on 15th December, 1967, that is, long before the judgment referred to above, was pronounced. Hence the question of revision under section 55 does not arise." * It is an against this order, the present writ petitions are filed. W.P. No. 153 of 1970 is for the issue of a writ of mandamus directing the 1st respondent to entertain the representation and deal with it as an application for rectification under section 55 of the Act. W.P. No. 152 of 1970 is for removal of the said order on the ground that it poses an error apparent. The main contention of the learned for the petitioner is that when the original order was made and even so when the appellate authority decided and concluded the appeal, there was no juridical pronouncement which clearly laid down the modus operandi under which raw hides and skins and dressed hides and skins should be taxed. or power which could be exercised according to the whims and fancies of such a statutory authority. If circumstances do exist for the exercise of power, it ceases to be a discretionary power but transforms itself into a mandate by statute to exercise that power in public interest and particularly for the avoidance of injustice in taxation matters. A subject cannot be lightly burdened with a tax if he can legally and lawfully avoid it and particularly by having recourse to the specified statutory authorities requesting them to rectify an error so as to avoid an illegal assessment; then it cannot be said that even in such a situation the word "may" has to be understood as "may"; on the other hand, it should be understood as "shall". It is this view which reflects in the judgment of the Supreme Court in Hirday Narain v. Income-tax Officer Quoting the well-known passage of Lord Chancellor Cairns in Julius v. Bishop of Oxford ( 1880 (5) AC 214 (H.L.)), the Supreme Court said that "exercise of power to rectify an error apparent from the record is conferred upon the Income-tax Officer in aid of enforcement of a right. The Income-tax Officer is an officer concerned with assessment and collection of revenue, and the power to rectify the order of assessment conferred upon him is to ensure that injustice to the assessee or to the revenue may be avoided. It is implicit in the nature of the power and its entrustment to the authority invested with quasi-judicial functions under the Act, that to do justice it shall be exercised when a mistake apparent from the record is brought to his notice by a person concerned with or interested in the proceeding." * If, therefore, the power vested in the authorities in sub-section (1) of section 55 is not always a discretionary power, but is normally to be understood as a duty with which the concerned authority is enjoined, then the next question is when shall the power be exercised. The learned Government Pleader would say that the interception of a judicial pronouncement between the concluded appellate, original or revisional order and the memorandum for rectification would not enable the authorities, even though bound to do so, to exercise such a power. Here again there is a fallacy. Of course, at one time this court took the view that such supervening judgment of courts, whether of the High Court or the Supreme Court, cannot by themselves form the foundation for the exercise of power to rectify an assessment order when the appropriate authority is asked to do so. But, in view of the decision in S. A. L. Narayana Row, Commissioner of Income-tax v. Model Mills Nagpur Ltd. it is difficult to hold that view. The Supreme Court in that case was considering the scope of the power of rectification vested in the Income-tax Officer under section 35 of the Income-tax Act. The short facts are : The respondent-company was subjected to, among other things, pay an additional tax on the excess dividends declared by it. The order was complied with. Thereafter, in Khatau Makanji Spinning & Weaving Co. Ltd. v. Commissioner of Income-tax