The cranial buckler of the _Dipterus_ (fig. 19) was, like that of the _Diplopterus_, of great beauty. In some of the finer specimens, we find the enamel ornately tatooed, within the more strongly-marked divisions, by delicately traced lines, waved and bent, as if upon the principle of Hogarth; and though the lateral plates are numerous and small, and defy the homologies, we may trace in those of the central line, from the snout to the nape, what seem to be the representatives of the frontal, parietal, and occipital bones,—the parietals ranging, as in the skull of the carp and in that of most of the mammals, in their proper place in the medial line. But the under surface of the cranium, armed, as on the upper surface, with plates of bone, exhibited an arrangement still more peculiar, (fig. 20.) Its rectangular patches of palatal teeth, its curious dart-like bone, placed immediately behind these, and attached, as the dart-head is attached to the handle, to a broad lozenge-shaped plate, with two strong osseous processes projecting on either side, forms such a _tout ensemble_ as is unique among fishes. Even here, however, there may be traced at least a shade of homological resemblance to the bones which form the base of the osseous skull. The single lozenge-shaped plate, (A,) with its dart-head, occupies the place of the basi-occipital bone; the posterior portion of the vomer seems represented by a strong bony ridge, extending towards the snout; two separate bones, each bearing one of the angular patches of teeth, corresponds to the sphenoid bone and its alæ; and attached laterally to each of these there is the strong projecting bone, on which the lower jaw appears to have hinged, and which apparently represents the lower part of the temporal bone. Not less singular was the form of the creature’s under jaw, (fig. 21.) I know no other fish-jaw, whether of the recent or the extinct races, that might be so readily mistaken for that of a quadruped. It exhibits not only the condyloid, but also the coronoid processes; and, save that it broadens on its upper edges, where in mammals the grinders are placed, so as to furnish field enough for angular patches of teeth, which correspond with the angular patches in the palate, it might be regarded, found detached, as at least a reptilian, if not mammalian, bone. The disposition of the palatal teeth of the _Dipterus_ will scarce fail to remind the mechanist of the style of grooving resorted to in the formation of mill-stones for the grinding of flour; nor is it wholly improbable that, in correspondence with the rotatory motion of the stones to which the grooving is specially adapted, jaws so hinged may have possessed some such power of lateral motion as that exemplified by the human subject in the use of the molar teeth. [Illustration: Fig. 21. UNDER JAW OF DIPTERUS.]