hollows being analogous to the tooth-pits in the former tribe, and its protruded teeth to the similarly exposed teeth of the latter. [Footnote 47: I have seen detached teeth, wherein the length of the root, or part imbedded in the jaw, tripled that of the exposed part, sinking four or five inches into the bone without any trace of anchylosis. Whether these huge tusks belonged to the upper or under maxillary, I do not pretend to say, though no specimen of the under jaw, which has ever come under my notice, would accommodate half of such a deep-sunk base.] [Illustration: Fig. 31.--Jaw of Holoptychius (_Rhizodus_.--Owen) from Gilmerton, one-fourth nat. size; the large teeth along the middle part of the jaw are here wanting.] When we bring the microscope to bear upon the elucidation of the structure of these ancient teeth, it seems as if our labour had but just begun; and that so far from having by an external scrutiny exhausted all that they have to show us, our knowledge of them can be but scanty and superficial until we have studied them carefully under a magnifying power. Microscopic sections of such organic remains are prepared in the same way as those of the fossil woods already noticed; and a more interesting or beautiful series of objects cannot be conceived than a set of slices of these fossil-teeth. Viewed, then, in longitudinal section from base to point, the part above the fluted root of one of the large teeth of the holoptychius is seen to consist of minute hair-like fibres of extreme tenuity, which proceed in straight lines from the outer surface to the interior. At right angles to these, and parallel with the outer edges, there is a set of dark widely-placed lines conforming to the outline of the tooth, like so many long sugar-loaf shaped caps, placed within each other. When this part is cut across, and viewed in transverse section, the tooth is observed to be of a flattened oval form, with the same fine fibres or tubes radiating from the centre, and traversed by the same dark bands which now assume the form of concentric rings. The appearance thus presented reminds one at once of a cross section of some dicotyledonous tree, the dark bands resembling the annual layers of growth, and like these resulting from a similar thickening of the internal tissue. The upper part of the tooth is solid and the concentric rings few; the middle exhibits an increase of the rings, and possesses, moreover, a hollow centre or pulp-cavity,[48] with the usual diverging fibres. Here the oval form is well shown, and the encircling rings are considerably flattened at the ends of the long axis. [Footnote 48: This hollow centre may be seen occasionally filled up by a sharp conical tooth like the _phragmocone_ of a belemnite.]