The building material at Firûzâbâd is undressed stone, very roughly coursed and set in a bed of mortar. In the domes the stones are cut thinner, more carefully coursed and provided at intervals with a bonding course; in the vaults the thin slabs are laid vertically, parallel with the main axis of the chamber. Exactly the same principles are observed at Ukhaiḍir. Nor do the resemblances end here. Tubes are not absent from the vaulting system,[134] and most of the archways are set back from the jambs to facilitate the placing of centering.[135] The arches are semicircular as at Qaṣr-i-Shîrîn. In the vault of the big lîwân centering would seem to have been used, for it is set back from the face of the walls, doubtless in order to leave a convenient ledge for the centering beams. The vaults and domes here and in all other Sasanian buildings have the ovoid shape common to Ukhaiḍir and to subsequent Mohammadan work in Mesopotamia. It is the old Mesopotamian vault contour. The exterior walls of Firûzâbâd are broken into a continuous series of recessed and arched blind niches divided by engaged columns carrying an entablature of modest proportions.[136] The appearance of this decoration is to my eyes so entirely un-Hellenistic that I have difficulty in connecting it with any classical influence, and in point of fact an arched niche from one of the reliefs from Quyundjik, in the British Museum (Fig. 11), is nearer akin to it than such [Illustration: FIG. 11. Relief from Quyundjik. (From _L’Art antique de la Perse_, by kind permission of M. Dieulafoy.)] façades as those of Ctesiphon or Ukhaiḍir. But it must be admitted that while the recessing of Babylonian and Assyrian wall surfaces is in no sense an imitation of architectural forms, least of all an imitation of the column, which was an element unknown to the designers of these recessed buildings,[137] and that while on the Quyundjik relief the architrave is placed directly upon the piers without the intermission of impost or capital, the engaged columns of Firûzâbâd are true columns carrying an impost, and the whole scheme is no longer a pattern, but a copy in relief of a colonnade in the round. In the courtyard the rectangular niching is retained, but without the engaged columns.[138] On the façade of the palace a series of seven arched niches is set high up in the wall, on either side of the arched opening of the lîwân.[139] It is a motive which recalls the open loggias in the façade of an Assyrian palace.[140]