A Christian church, in its early form, has been familiarly described as the pagan temple turned inside out. To convert the ideal temple into the ideal church, the wall and the colonnade must change places. So completely is this the case that some of the earlier commentators on Vitruvius were completely puzzled between the wall of the temple and the peristyle. They assumed that the latter _must_ be _within_ the wall, as in their own churches, and based their remarks on this supposition. The comparison between these almost contemporary works, the Temple of Jupiter in Diocletian’s Palace at Spalatro, and the tomb of St. Constantia at Rome, exactly illustrates this change. In the one the solid wall forms the circle and carries the dome, and the colonnade is external (See Fig. 403), in the other the colonnade forms the inner circle and carries the dome, while the wall becomes external, an _aisle_ taking the place of the peristyle (See Fig. 404). The colonnade is _doubled_ to support the massive clerestory whence the dome springs, and the whole assumes the type of one form of Christian church, which henceforth became of frequent occurrence. The dome, in this instance, ceases to be an external feature, being covered over by a conical roof. I may add that the peristyle is repeated in the old manner as an additional feature beyond the wall of the aisle. The baptistery at Nocera is similar in distribution to the tomb of St. Constantia, and may be of similar date. It is ruder, however, and loses much beauty by the omission of the clerestory, and the admission of light through the haunches of the dome. Among the innumerable remains of domes of the older type, I will only mention one more, before proceeding to the second branch of my subject, to which it, in fact, properly belongs. That to which I allude is the baptistery at Ravenna, erected, as it is supposed, about the year 450 (Fig. 405). This is a very charming building, octagonal in form, yet covered over by a hemispherical dome. Though having no surrounding aisle, the design of its sides seems derived from the aisle and clerestory: indeed, it has a clerestory, though the arcade below is rather rudimentary than real. The dome, like that last alluded to, is covered externally by a sloping roof. The special feature, however, in this dome is that it rests upon an octagonal wall, or rather upon eight arches. We shall presently see how this was effected in subsequent times, and I will not anticipate that subject, but will content myself with mentioning that this seems to anticipate the Byzantine domes of the succeeding century, as had been the case in two other instances to which I shall have to refer, and as had been _nearly_ the case in the Temple of Minerva Medica. [Illustration: Fig. 405.--Baptistery at Ravenna. Plan and Section.]