The church of S. Giorgio in Velabro, which stands between the Palatine Hill and the river near the Piazza Bocca della Verità, retains the ancient name of this district, formerly a swamp called the Velabrum. This is perhaps the best point from which to begin our survey of the ruins of the Velabrum. The most conspicuous ruin near the church is the archway called Janus Quadrifrons, from its quadrilateral shape. It is a massive square building of white marble, with four piers supporting as many arches which are united in the centre, by a vaulted roof. Each pier has on the exterior twelve niches in two rows, with semicircular shell-shaped crowns. These two rows of niches were formerly separated by a projecting cornice which is now nearly destroyed except in the interior. The niches nearest to the corners on the north and south sides are not hollowed out, but only traced on the exterior surface, in order not to endanger the solidity of the angles. The present height of the building is thirty-eight feet, but it probably had an attica originally upon the top to which the staircase still extant led, and in which were some small rooms for the transaction of business. Upon the key-stones of the arches two figures can be still recognized, one of Rome and the other of the patroness of trade, Minerva. The exterior surface was doubtless decorated with rows of Corinthian columns between the niches, a large quantity of remains of such columns having been found in clearing the base, and in the niches themselves statues of various deities probably stood. The purpose of this arch was probably solely ornamental, and it stood by itself in some part of the Forum Boarium. The rooms in the attica may have been used for the accommodation of some of the officials of the cattle market. The builder and date are alike unknown. From the style of its architecture and sculptures, it has been pronounced decidedly later than the age of Domitian, to whom from his fondness for building Jani, it might be attributed. Platner and Becker suggest that it is identical with an archway called the Arcus Constantini--in the catalogue of the eleventh region--but a comparison of the style of the remnants of sculptures upon it with those on the existing arch of Constantine, does not confirm this conjecture. [Sidenote: Arcus Argentinorum.]