The nave usually had three entrance doors, and the aisles one or more each. As the heathen religions, and consequently the ancient temples, fell into disuse, there was plenty of building materials ready formed and dressed, which the architects of the new buildings appropriated for their own purposes in the erection of the basilicas. This accounts for the great number of Roman Corinthian and Ionic columns found in the buildings of the early Christian architecture, and we often find that when an ancient column was too short, it was simply raised on a higher base, and if too long it was cut down to fit its new position. It was generally in the later basilicas that this occurred, as might be expected, for the earlier basilicas are richer and better decorated in their beautiful details, seeing that the early Christian builders had the first choice of the rich ornamental work of architectural sculpture that had belonged to the ancient Roman temples. The church of S. Apollinare in Classe at Ravenna may be cited as one of the most finished and most beautiful of the early basilicas, which was erected with much of this old material. Although the Christian architects and artists were slow in producing new forms of plastic art, as long as they could adapt the existing fragments of architectural sculpture to their uses; on the other hand, the art of painting and decorating by mosaic pictures on the great spaces of the walls and ceilings of the basilicas was developed to a high degree of monumental splendour, and brilliant effects were gained by the use of gold and bright colours. Mosaics as wall decoration in the basilicas were suggested by the paintings in the catacombs. These primitive paintings were borrowed in their form and essence from ancient mythological works. At first, some of the earliest efforts at decoration in the catacombs consisted merely of monograms and symbols, such as the Greek letters Alpha and Omega, and the initials or monogram of Christ. [Illustration: Fig. 332.—Painting from the Catacomb of S. Agnese.]