On the western side of the Syrian desert the evolution of the capital is different. The engaged capitals at Madâin Ṣâliḥ and Petra show a marked tendency towards the Corinthian. Like the capitals of the Kôm al-Shukâfa oasis[341] and capitals on Pompeiian frescoes of the second style, they have the Corinthian form and the Corinthian rosette upon the abacus, not indeed worked out into a true rosette, but left in the shape of a simple boss. In the second-century façades at Petra, such as the Corinthian tomb and the Khazneh, this tendency reaches full expression. The replacing of the architrave by the archivolt created a structural need which was satisfied by the introduction of the impost-capital, and we find the latter both at Mshattâ[342] and at Muwaqqar,[343] the capitals at Muwaqqar being closely related to the Bîsutûn-Iṣfahân type. With these stone-carved capitals, the brick and plaster capitals of Ukhaiḍir, so far as they are preserved, are little concerned. The further history of the Muwaqqar capitals must be sought, in the realm of Mohammadan art, at Sâmarrâ and in the mosque of Ibn Ṭulûn.[344] New to Mesopotamian architecture are the clustered columns in the middle zone of the Ukhaiḍir façade. No doubt they are not essentially different from the triple supports between the arches of the second zone at Ctesiphon; but at Ukhaiḍir they are given a true architectural meaning, the central pair carries the wall, the flanking columns carry the cusped arches; moreover they are set in different planes, the central pair standing in front of the flanking columns. The effect produced is almost Gothic, a foreshadowing of the clustered piers of Armenian churches.[345] It was a scheme which was not to remain sterile in early Mohammadan art. Clustered piers carried the roof of the great mosque at Sâmarrâ[346] and the arcades of the mosque at Ibn Ṭulûn.