The Foot-prints of the Creator

by Hugh Miller

Fig. 17.

CRANIAL BUCKLER OF DIPLOPTERUS.

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 trace, as I have said, has yet been detected. The solitary exception in the case is, however, one of singular interest.

Fig. 18.

a. Palatal dart-head.

b. Group of palatal teeth.

In a collection of miscellaneous fragments sent me by Mr. Dick from the rocks of Thurso, I detected patches of palatal teeth ranged in nearly the quadratures of circles, and which radiated outwards from the rectangular angle or centre, (fig. 18, b.) And with the patches there occurred plates exactly resembling the barbed head of a dart, (a,) with which I had been previously acquainted, though I had failed to determine their character or place. The excellent state of keeping of some of Mr. Dick’s specimens now enabled me to trace the patches with the dart-head, and several other plates, to a curious piece of palatal mechanism, ranged along the base of a ganoid cranium, covered externally by a brightly enamelled buckler, and to ascertain the order in which patches and plates occurred. And then, though not without some labor, I succeeded in tracing the buckler with which they were associated to the Dipterus,—a fish which, though it has engaged the attention of both Cuvier and Agassiz, has not yet been adequately restored. It is on an ill-preserved Orkney specimen of the cranial buckler of this Ganoid that the ichthyologist has founded his genus Polyphractus; while groupes of its palatal teeth from the Old Red of Russia he refers to a supposed Placoid,—the Ctenodus. But in the earlier stages of palæontological research, mistakes of this character are wholly unavoidable. The palæontologist who did avoid them would be either very unobservant, or at once very rash and very fortunate in his guesses. If, ere an entire skeleton of the Ichthyosaurus had turned up, there had been found in different localities, in the Liasic formation, a beak like that of a porpoise, teeth like that of a crocodile, a head and sternum like that of a lizard, paddles like those of a cetacean, and vertebræ like those of a fish, it would have been greatly more judicious, and more in accordance with the existing analogies, to have erected, provisionally at least, places specifically, or even generically separated, in which to range the separate pieces, than to hold that they had all united in one anomalous genus; though such was actually the fact. And Agassiz, in erecting three distinct genera out of the fragments of a single genus, has in reality acted at once more prudently and more intelligently than if he had avoided the error by rashly uniting parts which in their separate state indicate no tie of connection.

Fig. 19.