arrangement of the diagonals (Fig. 79) that the chief interest in this early ambulatory lies. If not unknown in bays of rectangular plan, this was probably a first attempt to apply these intersecting ribs to bays of trapezoidal shape, a problem especially difficult when these bays had two curved sides. The ambulatory was so narrow and the wall piers with the two wall arches extended so far into its width that the space actually to be covered was of such a plan that ribs directly from the one pier to that diagonally opposite would have intersected almost against the crown of the apsidal arch. To avoid this awkward arrangement, and make the panels of more equal size, the builders either timidly broke the line of the rib, as in the second bay from the southwest (Fig. 79), or curved the ribs slightly away from the crown of the apse arches as in the northwest bay. Whether the builders were actually experimenting here at Morienval with the position of the diagonals and whether this little work of the early twelfth century had any influence upon later ambulatory vaulting may be an open question, yet it is a fact that the later ambulatories with ribbed vaults over trapezoidal bays show three distinct types in the arrangement of the diagonals according as these are left straight in plan, or curved, or broken to bring their crowns to a better point in relation to the crown line of the enclosing arches. TRAPEZOIDAL AMBULATORY VAULTS WITH STRAIGHT DIAGONAL RIBS Of the three types, the one with straight diagonals (Plate III-a) is perhaps most seldom seen, probably because of the awkward place at which its vault crown falls. It does appear, however, in the cathedral of Aversa near Naples[426], where the heaviness of the ribs would seem to denote an early date.[427] There are a few later examples elsewhere, among [Illustration: PLATE III] them the cathedrals of Langres (Haute-Marne) (end of twelfth century) (Fig. 80) and Milan (beg. 1386), while a similar system with one or more added ribs in the outer severy is to be seen at Pontoise (Seine-et-Oise) S. Maclou (Plate III-h), in the cathedral of Rouen (Plate III-d), and in Saint Remi at Reims (Fig. 83), which are later described. [Illustration: FIG. 80.--LANGRES, CATHEDRAL.] 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). TRAPEZOIDAL AMBULATORY VAULTS WITH BROKEN RIBS