The roof is not strictly original, most of it having been rebuilt in 1822-1824, when, however, the old material was worked in again as far as possible, and the old quadripartite groining adhered to. It may be noticed that the vaulting is carried out very systematically and correctly, the only defect being that the wall-ribs die into the vaulting surfaces, instead of being brought down to the clerestory sill. The plough-share surfaces (as they are called) are nevertheless well cut back to concentrate the lateral pressures against the external buttresses. In the nave the new vaulting has the wall-ribs properly supported by light shafts in the angles of the clerestory openings, whilst in the transepts the inner archivolt of the windows answers the same purpose. [Illustration: _Photo._ _G.P. Heisch._ THE TRIFORIUM AND CLERESTORY OF THE CHOIR.] It is highly probable that the choir formerly extended to the western side of the tower, as indicated by the step between the nave and tower pavement. The =Altar-platform=, though raised seven steps above the nave pavement, gives the altar a rather low elevation as compared with the lofty Continental altars, whether abroad, or introduced here in recent years on the Continental example. Herein it exhibits a peculiarity of the English use, as illustrated in many pre-Reformation churches, where the occasional deviations from rule can generally be accounted for by the lofty crypt beneath, as, _e.g._, at Canterbury.[26] Behind the altar rises the magnificent =Screen=, erected by Bishop Fox in 1520, which almost fills the eastern end of the choir. This fine work had been more or less mutilated through the iconoclastic zeal of ultra-reformers, who deprived it of the sculptured figures in the niches. It was further ill-treated during the architectural supremacy of Sir Christopher Wren and his school, when the smaller canopies and other projections were pared off to make a level surface for the classical piece of woodwork placed in front of it. When this incongruous structure was removed and the restoration taken in hand (in 1833) by Mr. Wallace, liberties were again taken with the unfortunate screen, more or less spoiling the design, though undertaken on a good motive. Perhaps the least objectionable of these innovations was the insertion of panels for the Creed, Lord's Prayer, and Ten Commandments, in perpetuation of those in the wooden altar-piece, where the formulae had been set up in the spirit of the Injunctions of 1536 and 1538. Above the stages Mr. Wallace introduced rows of angels, the highest row being surmounted by a cornice of strawberry-leaf ornament for which there was no sort of precedent, either in the original work here, or in other altar-screens of similar character elsewhere.[27]