Frets of all types were common, and the acanthus rinceau, or running scroll, was frequent, drawn in bold silhouette, with the utmost simplicity, appropriate to the material. In similar technique were the silhouettes of dogs, with the inscription cave canem ("beware of the dog"), that occasionally decorated vesti bule floors. A third type comprised floors where the entire sur face was made a rich decorative composition, usually by means of bold geometric patterns of considerable intricacy. Frequently, small panels of pictures were placed in important positions. It is to this type that the finest examples of Roman floor mosaic belong, such as the remarkable series found in and near Carthage, with their simple representations of men fishing, and other similar genre subjects, and the great mosaic from the baths of Otricoli, now in the Sala Rotunda in the Vatican. The last class is that of the frankly pictorial mosaic, undoubtedly based on Greek pre cedent. The famous "unswept room," now in the Lateran Museum, with its litter of fish, vegetables and fruit, and its mouse gnawing a nut, was copied or adapted from a well-known Hellenistic original at Pergamum.
Another type of floor treatment was the large-scale marble mosaic, opus sectile. In this, large slabs of coloured marbles, cir cular or square, were used to form magnificent and simple all over patterns, usually with an alternation in adjacent units, of square and circular forms ; there was sometimes, also, an addi tional alternation in colour. The most famous example, unique in being still in place and in use after more than 1800 years, is the great marble floor of the Pantheon, at Rome (begun f fo), whose alternating squares and circles have been the inspiration for countless marble floors in modern days.
It is known from literary sources that mosaics, chiefly of glass, were much used also on the upper walls and vaults of monu mental buildings of the Roman empire. Unfortunately, few, if any, examples of vault decoration earlier than the 4th century remain, and none in situ. Pompeii, however, has furnished several instances of more informal uses, especially mosaic columns and several mosaic garden fountain niches. Certain small panels of fine scale, and exquisite draughtsmanship, such as the two panels of rabbits in the Metropolitan Museum, New York, may have been used on walls rather than floors. It is noteworthy that the Pompeiian columns and fountains already show certain features characteristic of later Byzantine mosaics, both in the roughness of the texture, and in the colouring, largely blue-green. Column
patterns seem to have been based on textile motives. The finest example of classic Roman vault mosaic is that in the aisle of S. Costanza (dated by Rivoira 324-326). Portions that remain are remarkably rich, varied and skilful in design; parts have leaves; birds and fishes are scattered over a light ground with lavish informality; and other parts show interlacing, geometric patterns. The most famous fragment represents a vintage scene, with the grape gatherers climbing through the vines, whose supporting lattice forms an interesting geometric basis.
In the 5th century mosaics of the tomb of Galla Placidia (c. and the baptistry of Neon (5th century), both at Ravenna, the rich mosaics covering walls and vaults still remain dominantly classic and Roman, although elements characteristic of later Byzantine work had appeared. Those of the baptistry, with their great swirling acanthus rinceaux in the lower arch spandrils, and the rich architectural ornament of columns and shell-topped niches, at the base of the dome, and the garland or wreath on the under side of the entrance arch of the tomb of Galla Placidia are especially important in showing what must have been the character of the monumental Roman mosaics of the earlier period.