These are petitions for writs of certiorari seeking to set aside an assessment on the petitioner of a part of the expense of the alleged alteration of two streets in the city of New Bedford in which its tracks had been located for five years or more. The assessments purport to have been laid under St. 1906, c. 463, Part III, § 68. (G. L. c. 161, § 79). It is required by this section that, in the total expense on which the proportionate share of the street railway shall be calculated, “ the necessary cost of changing its railway to conform to such alteration ” must be included. *106This statute is mandatory and not permissive. It confers substantial rights upon the street railway, upon which it .has a right to insist. It is conceded and appears on the face of the records of the city that no part of that “ necessary cost ” was included in the present assessment. It is agreed that the amount actually so expended was on one street about $9,600 and on the other about $2,000. These amounts are not insignificant and cannot be overlooked. The record contains no adjudication concerning the subject. The circumstance that before the orders were passed a letter was sent by the petitioner to the board of mayor and aldermen, stating the entire expense incurred by it in connection with the work, cannot be taken as matter of law to be a waiver of its substantial rights or to excuse the respondents from attempting to make a correct computation of the total expense. For this reason the assessment so far as concerns this petitioner was contrary to the plain terms of the statute. An assessment of this nature, like a tax, must stand or fall on its own merits. It cannot be corrected by a judicial review. It becomes unnecessary to consider the other grounds urged by the petitioner. In each case peremptory writ may issue declaring invalid the assessment against the petitioner.

So ordered.