It is roofed by a concha, or semi-dome; and the external arrangement of its walls provides for the security of this dome by what is, in fact, a system of buttresses as effective and definite as that of any of the northern churches, although the buttresses are obtained entirely by adaptations of the Roman shaft and arch, the lower story being formed by a thick mass of wall lightened by ordinary semicircular round-headed niches, like those used so extensively afterwards in Renaissance architecture, each niche flanked by a pair of shafts standing clear of the wall, and bearing deeply moulded arches thrown over the niche. The walls with its pillars thus forms a series of massy buttresses, on the top of which is an open gallery, backed by a thinner wall, and roofed by arches whose shafts are set above the pairs of shafts below. On the heads of these arches rests the roof. We have, therefore, externally a heptagonal apse, chiefly of rough and common brick, only with marble shafts and a few marble ornaments; but for that reason all the more interesting because it shows us what may be done, and what was done, with materials such as are now at our own command; and because in its proportions, and in the use of the few ornaments it possesses, it displays a delicacy of feeling rendered doubly notable by the roughness of the work in which laws so subtle are observed, and with which so thoughtful ornamentation is associated. We must now see what is left of interest within the walls. All hope is taken away by our first glance; for it falls on a range of shafts whose bases are concealed by wooden panelling, and which sustains arches decorated in the most approved style of Renaissance upholstery, with stucco roses in squares under the soffits, and egg and arrow mouldings on the architraves, gilded, on a ground of spotty black and green, with a small pink-faced and black-eyed cherub on every keystone; the rest of the church being for the most part concealed either by dirty hangings, or dirtier whitewash, or dim pictures on warped and wasting canvas; all vulgar, vain, and foul. Yet let us not turn back, for in the shadow of the apse our more careful glance shows us a Greek Madonna, pictured on a field of gold; and we feel giddy at the first step we make on the pavement, for it, also, is of Greek mosaic waved like the sea, and dyed like a dove’s neck.