Trapezoidal Ambulatory Vaults with Curved Diagonal Ribs
A little more common, perhaps, are the ambulatories with diagonal ribs of curved plan ([Plate III-b.]). The earliest existing example subsequent to Morienval would seem to be that of the abbey church at Saint Germer-de-Fly (cir. 1130-1150).[428] Its ribs correspond in general to the curve of the groins which would be produced by the intersection of a transverse with an annular tunnel vault. Such ribs are naturally difficult to construct because of their curvature in plan as well as in elevation and as a result they are but seldom found, though an example on a large scale appears in Bourges cathedral ([Plate IV-a.]).