Wall and ceiling mosaics from the classical period are rare. What appears to be the earliest example in Italy was identified in 1928 by G. Lugli at the villa of Hadrian. It is a floral-geometric design in black on a white ground of marble tesserae veined with a vitreous blue paste, which adorned the vaulted ceiling of a house of republican date, afterwards transformed by Hadrian into the imperial residence. Another example of the 2nd century A.D., first noted by Ashby (P.B.S.R., 1906, p. 104) is afforded by a circular domed nymphaeum near the Via Tiburtina, which is en tirely covered with a mosaic of plain white tesserae. But the de struction, partial or complete, of the imperial thermae and palaces has deprived us of the more elaborate means of passing judgment on the systems of opus musivum proper (see MosAic), i.e., the decoration of vaults and wall-surfaces with mosaics in glass, enamel or precious materials. We can, however, form some idea of these from the decoration of fountains at Pompeii and else where, as well as from the compositions which adorn the walls and apses of early Christian basilicas. The mosaic on blue ground of the god Silvanus, on a fountain niche from Ostia at the Lateran (Plate IV., fig. 8), shows a type of decoration whence derive the apsidal compositions of S. Pudenziana or of SS. Cosma and Dami ano. On the other hand, the celebrated groups of Justinian and Theodora with their courts, in S. Vitale at Ravenna, continue the series of those imperial groups of which the families of Maximian and of Constantine Chlorus, painted in a hall of the palace of Aquileia, afforded a noted and celebrated example. The "Roma Barberini" comes, as we have seen, from a similar group in Rome, and the Theodosius, with his sons and officers of State, on the silver plate in Madrid probably reproduced yet another. Fur ther fine and instructive examples of opus musivum may be studied at S. Costanza in Rome, built by Constantine early in the 4th century A.D. The mosaics of the cupola were destroyed in the
16th century, but those of the annular vault which surrounds the baptistry, though much restored, show that the decorative schemes of Erotes, vine-patterns, medallions, etc., commonly found in pavements were also used by the musivarii.
When employed in the service of Christianity the old pagan themes were invested with a new spiritual meaning, and mosaic became the chief channel through which motives invented by pa ganism were transmitted to Christian art. The acanthus decora tion in green and gold on blue ground in the baptistry of S. John Lateran continues the floral arabesques of the Ara Pacis, and sim ilar scrolls form the background of the arbor vitae at St. Clemente (time of Pope Paschal II., 1099-1118), and frame the central group of the apse mosaic at S. Maria Maggiore (time of Nicholas IV., 1288-92). In other mosaics we find the pagan river motive, with pleasure boats floating down stream and boys angling from the bank, to symbolize the river Jordan or the rivers of paradise.
Opus sectile, a marble intarsia of various colours, also deserves to be noted. An interesting example in Palazzo Colonna, possibly as early as the 1st century, represents the "Infancy of Romulus and Remus"; another, attributable to the 4th century, is provided by the magnificent, though sadly scattered remains of the revet ments in marble and mosaic of the basilica of Junius Bassus (con sul, A.D. 331) on the Esquiline. They are remarkable for brilliancy of colouring and for the magnificent frontal composition of cer tain of the panels; the one chosen for illustration (Plate IV., fig. I I), which represents the consul himself riding into the circus sur rounded by mounted attendants, has the same pictorial quality (the same absence, for instance, of ground line), as the Ravenna mosaics of Justinian and Theodora.