In proceedings to obtain a writ of certiorari the burden is upon the petitioner to establish all the material allegations of his petition. In this case, which was reserved upon the petition and answer, we must hold, in accordance with the decision in Dickinson v. Worcester, 188 Mass. 555, that the statements in the defendant’s official answer are to be taken as true, not only in those parts which set out the record, and the acts of the board within its jurisdiction which do not appear in the record, but also in those which allege extraneous facts which might have been traversed, and perhaps controlled by evidence. The affidavit appended to the petition could not have been received as evidence at a final hearing upon the facts, and cannot affect the rights of the parties upon this reservation.
*306The membership of the defendant board is not the same as when the assessment in question was made. But while its members change from time to time, the board itself as a tribunal is continuously the same. In Fairbanks v. Fitchburg, 132 Mass. 42, the board of mayor and aldermen that made the answer had not the same members as when it passed the order which was alleged to be illegal, but it was held that its right to answer was not thereby affected. In that case it appeared that the members who answered had executed the order which was called in question, and it was assumed that in the performance of that duty they must have known the facts connected with the adoption of the order. But the right of such a tribunal to answer as to its previous doings does not depend upon former action of the members who make the answer in the matter to which the answer relates. It will often happen that the doings of a board acting judicially will be first called in question after they have been completed, and after the membership of the board has entirely changed. In such a case, it is important that the proceedings should be officially set forth. And this can be done only by those who are members when the question arises. Their answer as a board is made over their own signatures, and under the sanction of their official oaths. As public officers they may be expected to act impartially. It may be presumed that, before making such an answer, they will acquire what may be called official knowledge of the acts of their predecessors. As members of the same board, they may in a variety of ways be expected to have opportunities of knowing what the board has done, and how it has done it. Without personal knowledge, they may have official knowledge, which is something better than mere hearsay. And in presenting an answer upon which the conduct of the board is to be passed upon by a higher tribunal, they should be permitted to state facts which they know officially, even though they do not know them personally. The statements in relation to the records of the board, and its doings within its jurisdiction, which appear in the answer in this case, seem to be of this kind. The others are mere allegations in the nature- of pleading, which may always be controverted by a petitioner. The form of jurat annexed to the answer does not indicate that those who answered had not official knowledge of the *307acts of the board in regard to which such an answer is ordinarily conclusive. It merely disclaims personal knowledge. But it is unnecessary to decide in this case whether any part of the answer except the record would have been conclusive if traversed by the petitioner, for, in accordance with the decision to which we have referred, all its allegations must be deemed to be true.
Although the proceedings which we are considering were very irregular, the legal questions which arise in the case as presented are comparatively simple. It is true, as contended by the petitioner, that the mayor and aldermen could not delegate the authority given them by the Pub. Sts. c. 50, § 1, to lay and make common sewers. But no suggestion is made that this sewer was not legally laid, and it is only objected that it was “ built under the supervision and direction of a committee, composed of four members of the common council and three aider-men.” But this was done by order of the mayor and aldermen. The statute which gave them authority to make the sewer did not preclude them from employing agents to supervise and direct the work. If there was any irregularity in their conduct in this respect, we cannot say that the sewer was not “ made ” by the defendants, or that it was illegally constructed.
The petitioner was not entitled to notice of the defendant’s intention to lay out and construct the sewer, or to make an assessment upon him. Allen v. Charlestown, 111 Mass. 123. Holt v. Somerville, 127 Mass. 408.
The validity of the assessment made by the mayor and aider-men is not affected by the fact that they called in another person to assist them in making it. Taber v. New Bedford, 135 Mass. 162.
The provisions of the ordinances requiring the superintendent of sewers to keep and submit to the board an account of the cost of constructing the sewer, and to report a list of the persons deriving benefit from it, were merely directory, and his failure to comply with them did not invalidate the assessment. Dickinson v. Worcester, 138 Mass. 555.
As a convenient method of equitably adjusting- the assessments, the members of the board might, if they saw fit, divide them “into three classes, direct benefit, remote benefit, and more remote benefit.”
*308After the assessments were made, the petitioner appears to have had notice of the assessment upon him, in accordance with the provision of the Pub. Sts. c. 50, § 4.
The allegations of the answer and the amendments to it cover all the other matters referred to in the petition, and we find that justice does not require that a writ of certiorari should issue.
Petition dismissed.