The second group is much larger and more widespread, and comprises all the churches employing transverse tunnel vaults over the side aisles. The examples belonging to the school of Perigord have already been discussed,[106] and mention has been made of the fact that there are possibly enough of such churches in Provence alone to constitute a fifth type in that school.[107] But the system is too widespread to be attributed to any one province. It is undoubtedly a product of Roman and very early mediaeval architecture, for it is to be seen in such buildings as the Basilica of Maxentius at Rome, and in a modified, ramping form at Aachen.[108] Its structural advantage lies in the large space which the tunnel vault affords for windows in the outer wall thus lighting both the nave and aisles. Among the many examples are the parish church of Chatillon-sur-Seine (Côte-d’Or)[109] of the twelfth century, the abbey churches of Hauterive (Savoie), Ronceray[110] (vaulted in 1115), Bénévent-l’Abbaye (Creuse),[111] and the cathedral of Lescar (Basses-Pyrénées),--in which, however, the vaults are an addition to a primitive construction.[112] In the church at Fontenay (Côte-d’Or)[113] (before the middle of the twelfth century) concealed flying buttresses appear over the transverse arches between the aisle bays, thus aiding in securing a more even abutment for the continuous thrust of the tunnel vault of the nave. A few churches like Cavaillon,[114] and the cathedral of Orange (Vaucluse),[115] have tunnel vaults over rectangular bays flanking the nave but not connected by arches to form side aisles. The vaulting of the ambulatory gallery of Mantes cathedral, of the aisles of Fountains Abbey in England, and possibly the original vaults of the aisles of Saint Remi at Reims[116] were also transverse tunnel vaults. These latter churches differ from the ones previously mentioned, however, in that they are not tunnel vaulted in the nave and, moreover, are constructed with a clerestory so that the side aisle vaults do not serve the purpose outlined in the account of tunnel vaulted churches in the preceding paragraph. TUNNEL VAULTS WITH CROSS RIBS This brings the discussion of the standard methods of tunnel vaulting to a close, but there remain two curious churches in which cross-ribs were added beneath the surface of simple tunnel vaults. One of these is at Lusignan (Vienne),[117] and the other at Javarzay (Deux-Sèvres). Both date from about 1120 to 1140 though the ribs may be a later addition to give the appearance of ribbed vaulting which was introduced at about this time. NAVES WITH GROINED VAULTS