some of these, as for example in the apse of La Madeleine at Vézelay, and those of the cathedrals of Soissons (Fig. 67) and Chartres (Fig. 68), the curve of the vault corresponds with this window-head arch, but in many other apses such as those of Bourges cathedral (Fig. 76), of Saint Étienne at Caen (Fig. 70), and of the Sainte Chapelle at Saint Germer, the builders without hesitation curved their vault surface away from the line of the window-head which would otherwise be the natural wall rib.[367] Although from the preceding facts, it would seem evident that the wall rib was not an essential structural member of the Gothic vaulting system it may have been of advantage in many instances for holding a temporary wooden centering during the construction of the vault panels. [Illustration: FIG. 64.--REIMS, SAINT REMI.] RADIATING-RIBBED CHEVETS CONTINUED Returning again to the radiating-ribbed chevet, especially that of Saint Germer-de-Fly (Fig. 63), it is important to note the one great weakness which this vault possesses. It lies in the position of the radiating ribs which abut the apsidal arch at its crown, in other words at a point not at all suited to meet the pressures which are thus brought to bear against it. A rather heavy arch between the apse and the remaining bay of the choir, though no heavier than those in the vaulted bays of the nave, aids in resisting the pressure but nevertheless such a vault is not strictly logical from a structural standpoint. It is not as well buttressed, for example, as the ribbed half dome of Saint Georges at Boscherville (Fig. 61), or the transept chevet at Tournai (Fig. 53), in which a tunnel vaulted bay precedes the arch against whose crown the radiants are brought to bear. It is not surprising that this vault was but little used in subsequent Gothic architecture. It is possible, however, to cite a few examples, among them the cathedrals of Séez (Orne) (end of the thirteenth century), Cambrai (Nord) (cir. 1250), and Dinan (Côtes-du-Nord) (end of the thirteenth century), the cathedral of Saint Sauveur at Bruges (Belgium) (probably thirteenth century), and the abbey church of Moissac (Tarn) (probably fourteenth century). There is also a peculiar form in which the ribs are narrowed toward the crown, in Santa Maria sopra Minerva at Rome (after 1285). Two other slight variants of the type, one in the church of Saint Pierre-le-Guillard at Bourges and the other in the cathedral of Moulins are later discussed. BROKEN-RIBBED CHEVETS