With the exception of the small palace chapels at Balkuwârâ, all the Mesopotamian mosques were laid out on the same plan, but they differed in details of construction. When columns were available they were used in the riwâqs, as at Kûfah and in the first mosque at Raqqah. Elsewhere there were either wooden columns (Baṣrah, Baghdâd), or columns of masonry (Ukhaiḍir); or the riwâqs might be built with brick piers (Manṣûr’s mosque at Raqqah, Abû Dulaf) or, where stone was easy to obtain, with stone piers (Ḥarrân). At Sâmarrâ there is an isolated example of a composite pier. The roofs also differ from one another. At Ḥarrân there must have been wooden gable roofs over the riwâqs; at Ukhaiḍir they were vaulted; at Abû Dulaf the flat wooden roof rested on arches; at Sâmarrâ, and probably at Baghdâd, it was carried directly by the piers or columns. The wide central aisle was present at Ḥarrân, at Sâmarrâ, and at Abû Dulaf; at the latter there is also a side transept, producing the same =⏊=-shaped plan that has been noticed in the Tunisian mosques. The data are too scanty to admit of any but the most general conclusion. We find divergent detail but no divergence in type, and the type in Mesopotamia, as in other parts of the Mohammadan world, was derived ultimately from the Prophet’s house at Medînah. It is in Mesopotamia that we have the earliest examples of the brick pier. We do not know how far Nûr al-Dîn’s reparations at Raqqah extended, nor what was the aspect of the façades of the ṣaḥn before his time, but at Abû Dulaf the original construction is preserved and the brick piers and arches of the façades bear in their spandrel niches a characteristic Mesopotamian trait. I do not doubt that the first Egyptian mosque built with brick piers, that of Ibn Ṭulûn, was inspired by the Mesopotamian scheme; the marks of relationship are too numerous not to be convincing. The engaged quarter-columns with which his piers are provided were no new thing. Engaged half-columns are universal at Ukhaiḍir, and the oblong piers with quarter-columns in Ibn Ṭulûn’s mosque are nothing but a translation into solid masonry, along the lines indicated at Ukhaiḍir, of the octagonal piers with angle colonnettes of Sâmarrâ. More than a hundred years later this building served as a model to al-Ḥâkim, and it is interesting to note that the Mesopotamian pier was applied at a still later date to a building which seems in other respects to have been a direct imitation of the Umayyad mosque at Damascus. The great mosque at Diyârbekr (I give here a plan which I made in 1911, Plate 90) is a patchwork of older materials re-used at different times. The oldest part of the existing structure is the west wing of the ḥaram, which is dated by an inscription in the year A.D. 1091,[426] but the west arcade of the ṣaḥn, though it is dated A.D. 1124, must preserve the memory of a plan which is older than that of the present mosque. It strikes the