[Illustration: FIG. 122.--SEPULCHRE AT CORNETO.] The other class of tombs are circular tumuli, similar to the Pelasgic tombs of Asia Minor; of these large numbers exist, but not sufficiently uninjured to enable us to restore them completely. They generally consisted of a substructure of stone, upon which was raised a conical elevation. In the case of the Regulini Galeassi tomb there were an inner and an outer tumulus, the latter of which covered several small tombs, while the inner enclosed one only, which had fortunately never been opened till it was lately discovered. This tomb was vaulted on the horizontal system--that is to say, its vault was not a true arch, but was formed of courses of masonry, each overhanging the one below, as in the Treasury of Atreus, and it had a curious recess in the roof, in which were found numerous interesting examples of Etruscan pottery. It is, however, clear from the city gates, sewers, aqueducts, &c., that the Etruscans were acquainted with and extensively used the true radiating arch composed of wedge-shaped stones (voussoirs), and that they constructed it with great care and scientific skill. The gate at Perugia, and the Cloacæ or Sewers at Rome, constructed during the reign of the Tarquins,[19] at the beginning of the sixth century B.C., are examples of the true arch, and this makes it certain that it was from the Etruscans that the Romans learned the arched construction which, when combined with the trabeated or lintel mode of construction which they copied from the Greeks, formed the chief characteristic of Roman architecture. The Cloaca Maxima (Fig. 123), which is roofed over with three concentric semicircular rings of large stones, still exists in many places with not a stone displaced, as a proof of the skill of these early builders. There are remains of an aqueduct at Tusculum which are interesting from the fact of the horizontal being combined with the true arch in its construction. [Illustration: FIG. 123.--CLOACA MAXIMA.]