[403] See Rivoira I, p. 184. [404] Now destroyed. See Rivoira I, p. 184. [405] Some examples of the walled off ambulatory are found, however. See Enlart, I, p. 234, note 5. [406] See Rivoira, I, p. 183, for dates of Santo Stefano and Ivrea. [407] An annular tunnel vault also covered the passage around the tribune of the so-called stadium of Domitian already mentioned. See Rivoira, I, p. 184. [408] For other examples, see Enlart, I, p. 266, note 6. A similar vault is sometimes found in the aisles of circular churches, as for example in Ste. Croix at Quimperlé (eleventh century). Baudot and Perrault-Dabot, II, pl. 5. [409] See Rivoira, II, p. 289. [410] At intervals this vault is cut by lunettes or groined bays but it is fundamentally an annular tunnel vault. [411] Exactly as has been suggested in regard to similar side aisle vaults. [412] See Enlart, I, p. 266, note 6. [413] Enlart, I, p. 34, Fig. 14. [414] Revoil, I, pl. XLVIII. [415] Baudot and Perrault-Dabot, II, pl. 25. [416] Rivoira, II, p. 270, Fig. 718. [417] See Rivoira, II, p. 8. See also the Duomo Vecchio at Brescia (Porter, Cons. of Lombard and Gothic Vaults, Fig. 49). [418] Porter, Cons. of Lombard and Gothic Vaults, Fig. 53. [419] Porter, Cons. of Lombard and Gothic Vaults, Fig. 52. [420] Enlart, I, p. 273, Fig. 105. [421] The date of this cathedral is uncertain and the exceptional character of its triforium leads to the suspicion that it may not now retain its original arrangement, though the writer has no proof of this suggestion. [422] In some instances these lintels have been cut through with an arch running up into the surface of the vault between the bays. [423] Previous to Porter there had been suggestions of this origin of the ribbed vault in Choisy’s work and in Rivoira’s Lombardic architecture, but their studies had been largely confined to vaults whose ribs were sunken into the masonry panels. [424] See discussion of this point on p. 136. [425] See p. 53, 54. [426] Plan in Rivoira, I, p. 222, Fig. 327.