Nevertheless it may have been a feeling on the part of the builders that there was a lack of abutment to the west of the keystone which led to the introduction of one or more short ribs at this point in a number of chevets of various dates throughout the Gothic era. Thus in the apse of Saint Étienne at Caen (Fig. 70),[381] of Saint Trophîme at Arles, and of the cathedral of Notre Dame at Mantes, a single rib runs out from the keystone of the chevet to that of the apsidal arch. (Plate II-h). Nor was this rib a continuation of a ridge rib in the choir, for in the instances just cited no such rib was employed. One is to be seen in a number of churches, among them such widely separated examples as San Saturnino at Pamplona,[382] Westminster Abbey,[383] and Saint Alpin at Chalons-sur-Marne.[384] All of these churches have diagonal-ribbed chevets, but there are instances of a short rib running to the apsidal arch even where the vault is of the buttressing ribbed type, as for example in the cathedral of Barcelona,[385] where it would seem to have been used to subdivide the great triangular transverse cell of the vault even more than to provide further apparent abutment for the other radiants (Plate II-i). Even in chevets of the first type with ribs radiating from the keystone of the apsidal arch, a rib is occasionally added in the bay preceding this vault, as for example in Saint Pierre-le-Guillard at Bourges (fifteenth century vaulting), where this short rib runs out only to the crown of the six-part vault with which the last bay of the choir is covered (Plate II-j). Occasionally, too, a church like the cathedral of Moulins (Allier) (1468-1508), with a ridge rib the length of the choir, is terminated by a chevet with radiating ribs which thus receive apparent abutment at their keystone (Plate II-k). [Illustration: FIG. 70.--CAEN, SAINT ÉTIENNE.] A similar purpose of providing apparent abutment would seem to account for the unusual form of the chevets of Bayeux cathedral (thirteenth century), and Sant’ Antonio at Padua (after 1232) in which all the radiants which ordinarily stop at the keystone are carried through against the face of the apsidal arch. At Bayeux there are two such ribs (Plate II-l)[386] and at Padua, three (Plate II-m). The latter is also exceptionally interesting in the form of its chevet which is really a combination of the diagonal and the buttressing ribbed type. Although there are occasional instances like the one at Barcelona, in which the transverse severy of a buttressing ribbed chevet is subdivided only by a ridge rib, it is far more common to find a more extensive subdivision of this bay when such subdivision was undertaken at all. Moreover, it is an interesting fact that many of the elaborated chevet vaults--for it may be noted here that the apse vault was elaborated just as were those in the remainder of the church edifice--are fundamentally based upon the simple chevet with buttressing ribs.