These cranial bucklers, extending from nape to snout, protected the exposed upper surface of the cartilaginous skull, and conformed to it in shape, as a helmet conforms to the shape of the head, or a breast-plate to the shape of the chest. And as the cartilaginous heads resembled in general outline the osseous ones, the buckler which covered their upper surface resembled in general outline the upper surface of the osseous skull. It was in no case entirely a flat plate; but in every species rounded over the snout and in most species at the sides; and so, in order that its characteristic proportions might be preserved throughout the various stages of growth in the head which it covered, it had to be formed from several distinct centres of ossification, and to extend in area around the edges of the plates originated from these. The workman finds no difficulty in adding to the size of a piece of straight wall, whether by heightening or lengthening it; but he cannot add to the size of a dome or arch, without first taking it down, and then erecting it anew on a larger scale. In the domes and arches of the animal kingdom, the problem is solved by building them up of distinct pieces, few or many, according to the demands of the figure which they compose, and rendering these pieces capable of increase along their edges. It is on this principle that the Cystidea, the Echinidæ, the Chelonian carapace and plastron, and the skulls of the osseous Vertebrata, are constructed. It is also the principle on which the cranial bucklers of the ancient Ganoids were formed.[17] And from the general resemblance in figure of these bucklers to the upper surface of the osseous skull, the separate parts necessary for the building up of the one were anticipated, by many ages, in the building up of the other; just as we find external arches of stone which were erected two thousand years ago, constructed on the same principle, and relatively of the same parts, as internal arches of brick built in the present age. Doubtless, however, with this mechanical necessity for correspondence of parts in the formation of corresponding erections, there may have mingled that regard for typical resemblance which seems so marked a characteristic of the _style_, if I may so express myself, in which the Divine Architect gives expression to his ideas. The external osseous buckler He divided after the general pattern which was to be exemplified, in latter times, in the divisions of the internal osseous skull; as if in illustration of that “ideal exemplar” which dwelt in his mind from eternity, and on the palpable existence of which sober science has based deductions identical in their scope and bearing with some of the sublimest doctrines of the theologian. “The recognition,” says Professor Owen, “of an ideal exemplar for the vertebrated animals, proves that the knowledge of such a being as man existed before man appeared; for the Divine mind which planned the