Removal from the Senate belonged to the Emperor either as censor, when he exercised the discretionary moral judgment which had been associated with the Republican _lectio_,[1778] or in virtue of that power of revision which, as we have seen, became associated with the Principate.[1779] The chief grounds of exclusion were lack of the requisite census, refusal to take the oath _in acta Caesaris_ which was demanded of senators as of magistrates,[1780] or condemnation for crime. The Senate itself, in the exercise of its judicial power, could add to the sentence which it inflicted on a senator the penalty of expulsion from the house;[1781] it might even make this expulsion a punishment for calumnious accusation.[1782] The revised list of the Senate (_album senatorium_) was posted up publicly every year,[1783] and the Emperor appeared at the head of this list as _princeps senatus_.[1784] The number of the Senate was fixed by Augustus at 600,[1785] and, as there seems to have been little or no alteration in the number of the quaestors, the size of the body into which they passed may have been fairly constant. Augustus also instituted fixed days for meeting. These regular meetings (_senatus legitimi_) took place twice a month, on the Kalends and the Ides, except during the autumn months of September and October, and attendance on these days was compulsory.[1786] Even to these meetings, however, there was a summons through an edict.[1787] Extraordinary sittings (_senatus indicti_) could also be held whenever the magistrate deemed them necessary.[1788] The presidency and summons belonged chiefly to the consuls, but, as in the Republic, were possessed also by the praetors and tribunes.[1789] When the Senate had been summoned, the Princeps shared in the presidency as a magistrate, and it is very questionable whether he ever appeared at the board in the character of a simple senator.[1790] As a magistrate he might address the house at any moment, and, during the early Principate at least, custom dictated that there should be a pause at the opening and at the close of a debate which the Princeps might fill up with an expression of opinion if he pleased.[1791] We have already noticed the singular privileges which he possessed in the matter of bringing business before the house.[1792]