The church of SS. Cosma and Damiano which now stands at the north-western end of the Basilica of Constantine is, like many other churches in Rome, constructed on the ruins of some ancient temple. A most careful account has been given of its various stages of construction by Mr. Parker.[38] This may possibly have been the site of the original Temple of the Penates, and some archæologists have thought that the round temple which has been converted into the vestibule of the church was that of the Penates. The doorway of this temple with its columns, frieze and bronze doors, has been raised from the lower level of the old temple, on which the crypt of the church now stands beneath the floor. At the back of this the nave of the church was built on the ruins of some unknown temple, and behind the apse of the nave is said by Mr. Parker to have been the wall on which the celebrated Capitoline marble plan of Rome was hung. The fragments of this plan, which was shaken down, he thinks, by the fall of a great mass from the Basilica of Constantine, were found here. The chapel of the Penates was on the road from the summa Sacra Via to the Carinæ, which would place it near this spot.[39] The name “Temple of Romulus” given to the ruins by mediæval writers may have been derived from some restoration by Romulus, son of Maxentius. [Sidenote: Lavacrum of Heliogabalus.] We now pass through the arch of Titus along the ancient road towards the Coliseum, and on the slope of the Palatine to our right stand the ruins of a mediæval church which was excavated in 1873 by Cav. Rosa. This is said to have been called the church of S. Maria Antiqua, and to have been placed upon the ruins of the lavacrum of Heliogabalus.[40] The chambers now disclosed seem to have been used as a bath. Along the side of the hill near this are numerous ruins similar to those on the other side of the Palatine, which were apparently guard rooms. CHAPTER II. THE FORUM ROMANUM. [Sidenote: Temple of Castor.] At a short distance from the entrance to the Palatine we can enter the Forum near the ruins of an ancient temple, three columns of which are still standing. These three columns are among the most conspicuous and beautiful remains of ancient Rome. No doubt can now be felt that they belonged to the Temple of Castor and Pollux. The situation agrees with that which is pointed out by the Ancyræan inscription, and by the fact that Caligula made a passage from the Palatine Palace to this temple.