The earliest form of Roman masonry, consisting of rectangular tufa blocks placed in layers alternately parallel to and across the line of the wall, so as to bind the mass together firmly, may be best seen in the ancient fragments of the Servian wall on the Aventine and the Quirinal Hills and in the ruins on the western slope of the Palatine. This kind of building is the natural product of the peculiar parallel cleavage in the tufaceous rocks of the Roman hills. In those parts of the Campagna where basalt rather than tufa becomes the usual material, as at Præneste, we find polygonal masonry. One specimen of a mode of construction anterior to the introduction of the arch into Roman masonry is left us at Rome. This is the vault of the old well-house near the Capitol called the Mamertine Prison, where we find overlapping horizontal blocks of stone which originally met in a conical roof, but are now truncated and capped by a mass of stones cramped together with iron. That the principle of the arch was known in the regal period of Rome is shown by the great arch of the Cloaca Maxima. But no arches remain of so early a date which are not subterranean, and it is not likely that the arch was used in the early temples at Rome. These were, as we learn from Vitruvius, constructed in the so-called Tuscan style, which was the Italian contemporary of the Greek Doric. It is possible that the columns in the walls of S. Maria in Cosmedin, which are placed at unusual distances from each other, may have been an imperial restoration of the Temple of Ceres, after the old Tuscan fashion (Fig. 1). The next modification of architectural style, which is usually called, from the general influence of the Greek colonists on Latin art, the Tusco-Doric order, may be seen in the lowest range of columns and bases in the Theatre of Marcellus. The shaft of these columns is much more slender than in the Grecian Doric, and only partially fluted, if at all; while a cima recta is substituted for the echinus of the capitals (Figs. 2 and 3). The position of the triglyphs and the proportions of the cornice were also much changed, and the whole effect became less massive and bold than that of the Tuscan temples. [Illustration: ROME THE PRESENT WALLS AND THE WALLS OF AURELIAN]