The lower portion of the tooth exhibits a much more complicated texture. Externally it is marked by deep longitudinal furrows, that run down the enamelled sides and sink into the jaw. When cut across at this ribbed part, the tooth is found to present the most complex and graceful internal structure. The prominent ridges between the furrows are seen to be produced by crumpled folds of the substance of the tooth, which roll inwards towards the centre, coalescing with each other, and forming intricate groups of circling knots and folds. In some places they seem all but separated from each other into little circles, pierced with a central aperture, and recall the aspect of the upper layer in the scale of megalichthys. Each of these loops and folds presents a texture exactly similar to that of the upper part of the tooth. The same minute hair-like tubes, darkened and thickened in the long axis, radiate towards the centre; the same concentric bands run from centre to circumference; so that the lower part of the tooth seems, as it were, made up of a bundle of smaller teeth partially melted into each other. Between these loops and folds circular meshes frequently occur, and add to the complexity as well as the beauty of the whole structure. One of these sections, with all its twisting crumples, and folds, and knots, and coloured meshes, and encircled rings, bears no small resemblance to an antique polished table that has been cut out of the gnarled roots of a venerable oak. This complex structure arose from the mode of growth of the tooth; each prominent external ridge continually turning inwards down the furrow on either side, and mingling in freakish knots with the folds that had gone before.[49] [Footnote 49: For an acquaintance with the remarkable teeth of this ancient fish, more minute than it had been my good fortune to possess before, I am indebted to a most interesting series of microscopical preparations kindly lent me from his extensive collection by my friend Mr. Alexander Bryson of Edinburgh.]