the second class should be called, and that they seldom descended so low as to come down to the lowest class.  Nor need we be surprised, that the present order of things, which now exists, after the number of the tribes was increased to thirty-five, their number being now double of what it was, should not agree as to the number of centuries of juniors and seniors with the collective number instituted by Servius Tullius.  For the city being divided into four districts, according to the regions and hills which were then inhabited, he called these divisions, tribes, as I think, from the tribute.  For the method of levying taxes ratably according to the value of property was also introduced by him:  nor had these tribes any relation to the number and distribution of the centuries.

The census being now completed, which he had brought to a speedy close by the terror of a law passed in reference to those who were not rated, under threats of imprisonment and death, he issued a proclamation that all the Roman citizens, horse and foot, should attend at daybreak in the Campus Martius, each in his century.  There he reviewed the whole army drawn up in centuries, and purified it by the rite called Suovetaurilia,[43] and that was called the closing of the lustrum, because it was the conclusion of the census.  Eighty thousand citizens are said to have been rated in that survey.  Fabius Pictor, the most ancient of our historians, adds that that was the number of those who were capable of bearing arms.  To accommodate that vast population the city also seemed to require enlargement.  He took in two hills, the Quirinal and Viminal; then next he enlarged the Esquiline, and took up his own residence there, in order that dignity might be conferred upon the place.  He surrounded the city with a rampart, a moat, and a wall:[44] thus he enlarged the pomerium.  Those who regard only the etymology of the word, will have the pomerium to be a space of ground behind the walls:  whereas it is rather a space on each side of the wall, which the Etruscans, in building cities, formerly consecrated by augury, within certain limits, both within and without, in the direction they intended to raise the wall:  so that the houses might not be erected close to the walls on the inside, as people commonly unite them now, and also that there might be some space without left free from human occupation.  This space, which was forbidden to be tilled or inhabited, the Romans called pomerium, not so much from its being behind the wall, as from the wall being behind it:  and in enlarging the boundaries of the city, these onsecrated limits were always extended, as far as the walls were intended to be advanced.