The principal Roman type would seem to have been the annular tunnel vault. An excellent example is to be seen in the amphitheatre at Nîmes in which the builders have even employed transverse arches of stone beneath the vault of brick.[407] Similar in character, though later in date and without transverse arches, is the fourth century annular vault of Santa Costanza in Rome. It is not surprising, therefore, to find the annular tunnel vault in a number of the earliest Romanesque ambulatories as, for example, at Ivrea and in the lower story of Santo Stefano at Verona, both dating from the close of the tenth century, and somewhat later at Vignory in France and in the gallery of the Tower chapel in London.[408] The annular tunnel vault never became in any sense a popular form, however, probably because it necessitated an impost above the level of the apsidal arches and exerted a continuous thrust throughout its whole extent. It is more often to be found in crypts, as in Saint Wipertus near Quedlinburg (936)[409] and in Chartres cathedral (1020-1028)[410] where there were no structural problems of support, or else with its imposts lowered and cut by lunettes into an interpenetrating form which is really an elementary groined vault and is later discussed. AMBULATORIES WITH HALF TUNNEL VAULTS Besides these annular vaults, there are a few examples of ambulatories with half tunnel vaults which may owe their origin to the desire of the builders to keep the outer impost of the vaults as low as possible and still raise the inner line above the apsidal arcade.[411] In any event such an ambulatory is occasionally found in churches where the aisles also are half-tunneled, as, for example, in the abbey church of Montmajour (cir. 1015-1018)[412] and in the twelfth century church of Saintes.[413] Though this type of vault apparently has no pre-Romanesque prototype, it is perhaps possible that the concentric aisle of the circular church of Rieux-Merinville (Aude) (eleventh century)[414] affords an earlier example of its use over a space of similar plan. There is also an interesting use of a half-tunnel vaulted triforium above the ambulatory and abutting the half dome of the apse which opens into it through five arches, in the church of Loctudy (Finistère) twelfth century.[415] There are, however, circular buildings of the Byzantine and Carolingian periods with vaulted aisles which may well have furnished the prototypes for other methods of ambulatory vaulting which the Romanesque builders employed. One of these is the Royal Chapel at Aachen (796-804), in which the aisles are two stories high with the lower story covered by groined vaults of alternately square and rectangular plan with no transverse arches separating the bays.[416] ROMANESQUE AMBULATORIES WITH ALTERNATING TRIANGULAR AND SQUARE BAYS