The cranial buckler of the _Diplopterus_ (fig. 16) somewhat resembled that of its fellow-dipterian the _Osteolepis_, but exhibited greater elegance of outline. My first perfect specimen, which I owe to the kindness of Mr. John Miller, of Thurso, an intelligent geologist of the north, reminded me, as it glittered in jet-black enamel on its ground of pale gray, of those Roman cuirasses which one sees in old prints, impaled on stakes, as the central objects in warlike trophies formed of spoils taken in battle. The rounded snout represented the chest and shoulders, the middle portion the waist, and the expansion at the nape the piece of dress attached, which, like the Highland kilt, fell adown the thighs. The addition of a fragment of a sleeve, suspended a little over the eye orbits, 2, 2, seemed all that was necessary in order to render the resemblance complete. But as I disinterred the buried edges of the specimen with a graver, the form, though it grew still more elegant, became less that of the ancient coat of armor; the snout expanded into a semicircle; the eye orbits gradually deepened; and the entire fossil became not particularly like any thing but the thing it once was,—the cranial buckler of the _Diplopterus_. The print (fig. 17) exhibits its true form. It consists of two main divisions, occipital (A) and frontal, (C, fig. 16;) and in each of these we find a pair of smaller divisions, with what seem to be indications of yet further division, marked, not by lines, but by dots; though I have hitherto failed to determine whether the plates which these last indicate possess their independent centres of ossification. Not unfrequently, however, has the comparative anatomist to seek the analogues of two bones in one; nor is it at least _more_ difficult to trace in the faint divisions of the cranial buckler of the _Diplopterus_, the homologues of the occipital, frontal, parietal, mastoid, and nasal bones, than to recognize the representatives of the carpals of the middle and ring finger in man, in the cannon bone of the fore leg of the ox. I may mention in passing, that the little central plate of the frontal division, (1, fig. 16,) which so nearly corresponds with that of the _Osteolepis_, occurred, though with considerable variations of form and homology, and some slight difference of position, in all the Ganoids of the Old Red Sandstone whose craniums were covered with an osseous buckler, and that its place was always either immediately between the eyes or a very little over them. Its never-failing recurrence shows that it must have had _some_ meaning, though it may be difficult to say what. In the _Coccosteus_ it takes the form of the male dovetail, which united the nasal plate or snout to the plate representative of the superior frontal. Of the cartilaginous box which formed the interior skull of either _Osteolepis_, or _Diplopterus_, or, with but one exception, of the interior skulls of any of their contemporaries, no