In all the cases which I have enumerated, I have supposed the result to be literally a _portion of the original dome_. As it happens, however, we have but few ancient examples of so strict an adherence to principle; though in modern works they are more frequent. The purest specimen I know (if I judge rightly from drawings) is the dome of the little church of SS. Nasario and Celso, at Ravenna, the Mausoleum of Galla Placidia, and of her two husbands, the Emperors Honorius and Valentinian II. This is a dome such as I described as standing between four walls, which intersect it in the form of arches. [Illustration: Fig. 409.] [Illustration: Fig. 410.] [Illustration: Fig. 411.] The next step beyond this was a very important one as to beauty of effect. I refer to the practice of drawing the circle by means of a moulding on the surface of the dome, touching the crowns of the arches. This is not only ornamental, but it has the effect of emphasising the first completed course of stones, and perhaps even of strengthening it, and it has the further effect of defining the spherical triangles between the arches which, when thus gifted with a separate existence, receive the name of “_pendentives_,” whence this entire class of domes are called “_pendentive_ domes.” The earliest specimen of this is probably the little tomb in the Via Nomentana near Rome (Fig. 412), which, though probably of earlier date than the church of SS. Nasario and Celso, carries out the pendentive principle to its full development, just as we see it treated at a later period in the double gate (Fig. 413) and the golden gate of the Temple Area at Jerusalem, most probably erected under Justinian. All these domes are segmental in section. [Illustration: Fig. 412. Tomb in the Via Nomentana, Rome.] [Illustration: Fig. 413.--Double Gate, Temple Area at Jerusalem.] The pure form, however, of a pendentive dome--that is to say, the form in which the pendentives and the upper portion are really veritable parts of one and the same original dome, in the plan or base of which the rectilinear figure is inscribed--was not long adhered to. It was soon felt that the disc enclosed by the circular moulding looked _flat_ and _ineffective_; and the idea early suggested itself of converting the circular moulding into a massive cornice, raising upon it a new dome of such proportions as should approve themselves to the eye, and allowing nought but the pendentives to remain of the original dome (Figs. 414, 415). [Illustration: Fig. 414.] [Illustration: Fig. 415.]