It had been long known to the continental naturalists, that in certain Russian deposits, very extensively developed, there occur in considerable abundance certain animal organisms; but for many years neither their position nor character could be satisfactorily determined. By some they were placed too high in the scale of organized being; by others too low. Kutorga, a writer not very familiarly known in this country, described the remains as those of mammals;—the Russian rocks contained, he said, bones of quadrupeds, and, in especial, the teeth of swine: whereas Lamarck, a better known authority, though not invariably a safe one,—for he had a trick of dreaming when wide awake, and of calling his dreams philosophy,—assigned to them a place among the corals. They belonged, he asserted, as shown by certain star-like markings with which they are fretted, to the Polyparia. He even erected for their reception a new genus of Astrea, which he designated, from the little rounded hillock which rises in the middle of each star, the genus Monticularia. It was left to a living naturalist, M. Eichwald, to fix their true position zoologically among the class of fishes, and to Sir Roderick Murchison to determine their position geologically as ichthyolites of the Old Red Sandstone.
Sir Roderick, on his return from his great Russian campaigns, in which he fared far otherwise than Napoleon, and accomplished more, submitted to Agassiz a series of fragments of these gigantic Ganoids; and the celebrated ichthyologist, who had been introduced little more than a twelvemonth before to the Pterichthys of Cromarty, was at first inclined to regard them as the remains of a large cuirassed fish of the Cephalaspian type, but generically new. Under this impression he bestowed upon the yet unknown ichthyolite of which they had formed part, the name Chelonichthys, from the resemblance borne by the broken plates to those of the carapace and plastron of some of the Chelonians. At this stage, however, the Russian Old Red yielded a set of greatly finer remains than it had previously furnished; and of these casts were transmitted by Professor Asmus, of the University of Dorpat, to the British and London Geological Museums, and to Agassiz. “I knew not at first what to do,” says the ichthyologist, “with bones of so singular a conformation that I could refer them to no known type.” Detecting, however, on their exterior surfaces the star-like markings which had misled Lamarck, and which he had also detected on the lesser fragments submitted to him by Sir Roderick, he succeeded in identifying both the fragments and bones as remains of the same genus and on ascertaining that M. Eichwald had bestowed upon it, from these characteristic sculpturings, the generic name Asterolepis, or star-scale, he suffered the name which he himself had originated to drop. Even this second name, however, which the ichthyolite still continues to bear, is in some degree founded in error. Its true scales, as I shall by and by show, were not stelliferous, but fretted by a peculiar style of ornament, consisting of waved anastomosing ridges, breaking atop into angular-shaped dots, scooped out internally like the letter V; and were evidently intermediate in their character between the scales which cover the Glyptolepis and those of the Holoptychius. And the stellate markings which M. Eichwald graphically describes as minute paps rising out of the middle of star-like wreaths of little leaflets, were restricted to the dermal plates of the head.
Agassiz ultimately succeeded in classing the bones which had at first so puzzled him, into two divisions—interior and dermal; and the latter he divided yet further, though not without first lodging a precautionary protest, founded on the extreme obscurity of the subject, into cranial and opercular. Of the interior bones he specified two,—a super-scapular bone, (supra-scapulaire,)—that bone which in osseous fishes completes the scapular arch or belt, by uniting the scapula to the cranium; and a maxillary or upper jaw-bone. But his world-wide acquaintance with existing fishes could lend him no assistance in determining the places of the dermal bones: they formed the mere fragments of a broken puzzle, of which the key was lost. Even in their detached and irreducible state, however, he succeeded in basing upon them several shrewd deductions. He inferred, in the first place, that the Asterolepis was not, as had been at first supposed, a cuirassed fish, which took its place among the Cephalaspians, but a strongly helmed fish of that Cœlacanth family to which the Holoptychius and Glyptolepis belong; in the second, that, like several of its bulkier cogeners, it was in all probability a broad, flat-headed animal; and, in the third, that as its remains are found associated in the Russian beds with numerous detached teeth of large size,—the boar tusks of Kutorga—which present internally that peculiar microscopic character on which Professor Owen has erected his Dendrodic or tree-toothed family of fishes,—it would in all likelihood be found that both bones and teeth belonged to the same group. “It appears more than probable,” he said, “that one day, by the discovery of a head or an entire jaw, it will be shown that the genera Dendrodus and Asterolepis form but one.” As we proceed, the reader will see how justly the ichthyologist assigned to the Asterolepis its place among the Cœlacanths, and how entirely his two other conjectures regarding it have been confirmed. “I have had in general,” he concluded, “but small and mutilated fragments of the creature’s bones submitted to me, and of these, even the surface ornaments not well preserved; but I hope the immense materials with which the Old Red Sandstone of Russia has furnished the savans of that country will not be lost to science; and that my labors on this interesting genus, incomplete as they are, will excite more and more the attention of geologists, by showing them how ignorant we are of all the essential facts concerning the history of the first inhabitants of our globe.”
I know not what the savans of Russia have been doing for the last few years; but mainly through the labors of an intelligent tradesman of Thurso, Mr. Robert Dick,—one of those working men of Scotland of active curiosity and well-developed intellect, that give character and standing to the rest,—I am enabled to justify the classification and confirm the conjectures of Agassiz. Mr. Dick, after acquainting himself, in the leisure hours of a laborious profession, with the shells, insects, and plants of the northern locality in which he resides, had set himself to study its geology; and with this view he procured a copy of the little treatise on the Old Red Sandstone to which I have already referred, and which was at that time, as Agassiz’s Monograph of the Old Red fishes had not yet appeared, the only work specially devoted to the palæontology of the system, so largely developed in the neighborhood of Thurso. With perhaps a single exception,—for the Thurso rocks do not yet seem to have yielded a Pterichthys,—he succeeded in finding specimens, in a state of better or worse keeping, of all the various ichthyolites which I had described as peculiar to the Lower Old Red Sandstone. He found, however, what I had not described,—the remains of apparently a very gigantic ichthyolite; and, communicating with me through the medium of a common friend, he submitted to me, in the first instance, drawings of his new set of fossils; and ultimately, as I could arrive at no satisfactory conclusion from the drawings, he with great liberality made over to me the fossils themselves. Agassiz’s Monograph was not yet published; nor had I an opportunity of examining, until about a twelvemonth after, the casts, in the British Museum, of the fossils of Professor Asmus. Besides, all the little information, derived from various sources, which I had acquired respecting the Russian Chelonichthys,—for such was its name at the time,—referred it to the cuirassed type, and served but to mislead. I was assured, for instance, that Professor Asmus regarded his set of remains as portions of the plates and paddles of a gigantic Pterichthys, of from twenty to thirty feet in length. And so, as I had recognized in the Thurso fossils the peculiarities of the Holoptychian (Cœlacanth) family, I at first failed to identify them with the remains of the great Russian fish. All the larger bones sent me by Mr. Dick were, I found, cerebral; and the scales associated with these indicated, not a cuirass-protected, but a scale-covered body and exhibited, in their sculptured and broadly imbricated surfaces, the well-marked Cœlacanth style of disposition and ornament. But though I could not recognize in either bones or scales the remains of one ichthyolite more of the Old Red Sandstone, “that could be regarded as manifesting as peculiar a type among fishes as do the Ichthyosauri and Plesiosauri among reptiles,”[8] I was engaged at the time in a course of inquiry regarding the cerebral development of the earlier vertebrata, that made me deem them scarce less interesting than if I could. Ere, however, I attempt communicating to the reader the result of my researches, I must introduce him, in order that he may be able to set out with me to the examination of the Asterolepis from the same starting-point, to the Cœlacanth family,—indisputably one of the oldest, and not the least interesting, of its order.
Fig. 2.
a. Shagreen of the Thornback (Raja clavata.)
b. Shagreen of Sphagodus,—a placoid of the Upper Silurian.[9]
So far as is yet known, all the fish of the earliest fossiliferous system belonged to the placoid or “broad plated” order,—a great division of fishes, represented in the existing seas by the Sharks and Rays,—animals that to an internal skeleton of cartilage unite a dermal covering of points, plates, or spines of enamelled bone, and have their gills fixed. The dermal or cuticular bones of this order vary greatly in form, according to the species or family: in some cases they even vary, according to their place, on the same individual. Those button-like tubercles, for instance, with an enamelled thorn, bent like a hook, growing out of the centre of each, which run down the back and tail, and stud the pectorals of the thorn-back, (Raja clavata,) differ very much from the smaller thorns, with star-formed bases, which roughen the other parts of the creature’s body; and the bony points which mottle the back and sides of the sharks are, in most of the known species, considerably more elongated and prickly than the points which cover their fins, belly, and snout. The extreme forms, however, of the shagreen tubercle or plate seem to be those of the upright prickle or spine on the one hand, and of the slant-laid, rhomboidal, scale-shaped plate on the other. The minuter thorns of the ray (fig. 2, a) exemplify the extreme of the prickly type; the fins, abdomen, and anterior part of the head of the spotted dog-fish (Scyllium stellare) are covered by lozenge-shaped little plates, which glisten with enamel, and are so thickly set that they cover the entire surface of the skin, (fig. 3, b,)—and these seem equally illustrative of the scale-like form. They are shagreen points passing into osseous scales, without, however, becoming really such; though they approach them so nearly in the shape and disposition of their upper disks, that the true scales, also osseous, of the Acanthodes sulcatus, (fig. 3, a,) a Ganoid of the Coal Measures, can scarce be distinguished from them, even when microscopically examined. It is only when seen in section that the distinctive difference appears. The true scale of the Acanthodes, though considerably elevated in the centre, seems to have been planted on the skin; whereas the scale-like shagreen of the dog-fish is elevated over it on an osseous pedicle or footstalk (fig. 5, a) as a mushroom is elevated over the sward on its stem; and the base of the stalk is found to resemble in its stellate character that of a shagreen point of the prickly type. The apparent scale is, we find, a bony prickle bent at right angles a little over its base, and flattened into a rhomboidal disk atop.