usage.  Everybody walked upon it:  the deep relief made it a receptacle for mud and rubbish.  The effigy of the deceased, as was probably intended by him, was humbled in the dust:  adhesit pavimento.  The slabs got injured, and were often protected by low tables with squat legs.  Later on the slabs were raised enough to prevent people standing on them, and thus became like free-standing tombs; but it only made them more suitable for the sitting requirements of the congregation.  These sunken tombs, in fact, became a nuisance.  Although they were not carved in the very deep relief like those one sees in Bavaria, they collected the dirt, and a papal brief was issued to forbid them—­ut in ecclesiis nihil indecens relinquatur,[107] and the existing slabs were ordered to be removed.  Irretrievable damage must have resulted from this edict, but fortunately it was disobeyed in Rome and ignored elsewhere.  Nowadays it has become the custom to place these slabs upright against the walls, thus preventing further detrition.  To Cavaliere D. Gnoli we owe the preservation of the Crivelli tomb, which was in danger of complete demolition.[108] By being embedded in a wall instead of lying in a pavement this kind of monument, while losing its primitive position, often gains in appearance.  Crivelli, for instance, lies within an architectural niche.  His head rests on a pillow, the tassels of which fall downwards towards his feet.  When placed against a wall the need for a pillow may vanish, but the meaning and use of the niche becomes apparent, while the tassels no longer defy the laws of gravitation.  He becomes a standing figure at once, and the flying putti above his head assume a rational pose.  It has been suggested that this and similar tomb-plates were always intended to be placed upright, and that the delicate ornamentation, of which some traces survive, would never have been lavished on marble doomed to gradual destruction.  No general rule can be laid down, but undoubtedly most of these slabs were meant to be recumbent.  There are few cases where some contradiction of emplacement with pose cannot be detected.  But two examples may be noted where the slabs were clearly intended to be placed in walls.  An unnamed bishop at Bologna lies down, while at either end of the slab an angel stands, at right angles to the recumbent figure, holding a pall or curtain over the dead man.[109] Signor Bardini also has an analogous marble effigy of a mitred bishop, about 1430-40, who lies down while a friar stands behind his head.  These slabs were, therefore, obviously made for insertion in a wall, and they are quite exceptional.  The tomb-plate of Bishop Pecci in Siena Cathedral is less open to objection on the ground of incongruity between its position and the Bishop’s pose.  It is made of bronze, and is set in the tessellated pavement of green, white and mauve marble.  Technically it is a triumph.  Although the surface is considerably