The arch round the eastern of the two doors that leads into corridor Q´ is surrounded by cusps (Fig. 94). (_Cf._ Ctesiphon, Dieulafoy, _op. cit._, vol. v. plate 6.) A blind arcade, borne by pilasters, is to be seen in courts M M´ and N N´. In the antechamber U there are shallow niches on either side of the doors. With regard to the date of Ukheiḍir there are three possible hypotheses. It may belong-- 1. To the Sassanian or Lakhmid period prior to the Mohammadan conquest. 2. To the 150 years after the Mohammadan conquest. 3. To the Abbâsid period, _i. e._ after A.D. 750. 1. In defence of the first theory can be urged the close relationship between Ukheiḍir and other places of the Sassanian age, not only in plan (_cf._ Ḳaṣr-i-Shîrîn, de Morgan, _Mission Scientifique en Perse_, vol. iv., part 2), but also in the technique of brick and stone masonry and in the principles of vault construction (_cf._ Ctesiphon, Firûzâbâd, and Sarvistân, Dieulafoy, _op. cit._). But since it is certain that the arts of the early Moslem era were dominated in Mesopotamia by Sassanian influence, these affinities do not offer a convincing proof of a pre-Mohammadan date. Even if Ukheiḍir belonged to the early Moslem age, it might, and probably would, have been built by Persian workmen. At the same time certain architectural features, such as the groined vault and the fluted dome, have not hitherto been observed in any Sassanian building. The earliest Mesopotamian example of the groined vault known to me, besides the groins of Ukheiḍir, is that of which fragments can be seen in the Baghdâd Gate at Raḳḳah.