a. Scales of Osteolepis macrolepidotus.
b. Scales of an undescribed species of Glyptolepis.[10]
(The single scales mag. two diameters;—the others nat. size.)
In the Dipterian genera Osteolepis and Diplopterus the scales were more unequivocally such than in the Acanths, and more removed from shagreen. The under surface of each was traversed longitudinally by a raised bar, which attached it to the skin, and which, in the transverse section, serves to remind one of the shagreen footstalk. They are, besides, of a rhomboidal form; and, when seen in the finer specimens, lying in their proper places on what had been once the creature’s body, they seem merely laid down side by side in line, like those rows of glazed tiles that pave a cathedral floor; but on more careful examination, we find that each little tile was deeply grooved on its higher side and end, (for it lay diagonally in relation to the head,) like the flags of a stone roof, (fig. 6, a,)—that its lateral and anterior neighbors impinged upon it along these grooves to the extent of about one third its area,—and that it impinged, in turn, to the same extent on the scales that bordered on it posteriorly and latero-posteriorly. Now, in the Cœlacanth family, (and on this special point the foregoing remarks are intended to bear,) the scales, which were generally of a round or irregularly oval form, (fig. 6, b,) overlapped each other to as great an extent as in any of the existing fishes of the Cycloid or Ctenoid orders,—to as great an extent, for instance, as in the carp, salmon, or herring. In a slated roof there is no part on which the slates do not lie double, and along the lower edge of each tier they lie triple;—there is more of slate covered than of slate seen: whereas in a tile-roof, the covered portion is restricted to a small strip running along the top and one of the edges of each tile, and the tiles do not lie double in more than the same degree in which the slates lie triple. The scaly cover of the two genera of Dipterians to which I have referred was a cover on the tile-roof principle; and this is an exceedingly common characteristic of the scales of the Ganoids. The scaly cover of the Cœlacanths, on the other hand, was a cover on the slate-roof principle;—there was in some of their genera about one third more of each scale covered than exposed; and this is so rare a ganoidal mode of arrangement, that, with the exception of the Dipterus,—a genus which, though it gives its name to the Dipterian sept, differed greatly from every other Dipterian,—I know not, beyond the limits of the ancient Cœlacanth family, a single Ganoid that possessed it. The bony covering of the Cœlacanths was farthest removed in character from shagreen, as that of their contemporaries the Acanths approximated to it most nearly; they were, in this respect, the two extremes of their order; and did we find the Cœlacanths in but the later geological formations, while the Acanths were restricted to the earlier, it might be argued by assertors of the development hypothesis, that the amply imbricated, slate-like scale of the latter had been developed in the lapse of ages from the shagreen tubercle, by passing in its downward course—broadening and expanding as it descended—through the minute, scarcely imbricated disks of the Acanths, and the more amply imbricated tile-like rhombs of the Dipterians and Palæonisci, until it had reached its full extent of imbrication in the familiar modern type exemplified in both the Cœlacanths and the ordinary fishes. But such is not the order which nature has observed;—the two extremes of the ganoid scale appear together in the same early formation: both become extinct at a period geologically remote; and the ganoid scales of the existing state of things which most nearly resemble those of ancient time are scales formed on the intermediate or tile-roof principle.
The scales of the Cœlacanths were, in almost all the genera which compose the family, of great size; in some species, of the greatest size to which this kind of integument ever attained. Of a Cœlacanth of the Coal Measures, the Holoptychius Hibberti, the scales in the larger specimens were occasionally from five to six inches in diameter. Even in the Holoptychius nobilissimus, in an individual scarcely exceeding two and a half feet in length, they measured from an inch and a half to an inch and three quarters each way. In the splendid specimen of this last species, in the British Museum, there occur but fourteen scales between the ventrals, though these lie low on the creature’s body, and the head; and in a specimen of a smaller species,—the Holoptychius Andersoni,—but about seventeen. The exposed portion of the scale was in most species of the family curiously fretted by intermingled ridges and furrows, pits and tubercles, which were either boldly relieved, as in the Holoptychius, or existed, as in the Glyptolepis, as slim, delicately chiselled threads, lines, and dots. The head was covered by strong plates, which were roughened with tubercles either confluent or detached, or hollowed, as in the Bothriolepis, into shallow pits. The jaws were thickly set with an outer range of true fish teeth, and more thinly with an inner range of what seem reptile teeth, that stood up, tall and bulky, behind the others, like officers on horseback seen over the heads of their foot-soldiers in front. The double fins,—pectorals and ventrals,—were characterized each by a thick, angular, scale-covered centre, fringed by the rays; and they must have borne externally somewhat the form of the sweeping paddles of the Ichthyosaurian genus,—a peculiarity shared also by the double fins of the Dipterus. The single fins, in all the members of the family of which specimens have been found sufficiently entire to indicate the fact, were four in number,—an anal, a caudal, and two dorsal fins; and, with the exception of the anterior dorsal, which was comparatively small, and bent downwards along the back, as if its rays had been distorted when young,[11] they were all of large size. They crowded thickly on the posterior portion of the body,—the anterior dorsal opposite the ventrals, and the posterior dorsal opposite the anal fin. The fin-rays of the various members of the family, and such of their spinous processes as have been detected, were hollow tubular bones; or rather, like the larger pieces in the framework of the Placoids, they were cartilaginous within, and covered externally by a thin osseous crust or shell, which alone survives; and to this peculiarity they owe their family name, Cœlacanth, or “hollow-spine.” The internal hollow, i. e. cartilaginous centre, was, however, equally a characteristic of the spinous processes of the Coccosteus. In their general proportions, the Cœlacanths, if we perhaps except one species,—the Glyptolepis microlepidotus,—were all squat, robust, strongly-built fishes, of the Dirk Hatterick or Balfour-of-Burley type; and not only in the larger specimens gigantic in their proportions, but remarkable for the strength and weight of their armor, even when of but moderate stature. The specimen of Holoptychius nobilissimus in the British Museum could have measured little more than three feet from snout to tail when most entire; but it must have been nearly a foot in breadth, and a bullet would have rebounded flattened from its scales. And such was that ancient Cœlacanth family, of which the oldest of our Scotch Ganoids,—the Asterolepis of Stromness,—formed one of the members, and which for untold ages has had no living representative.
Let us now enter on our proposed inquiry regarding the cerebral development of the earlier vertebrata, and see whether we cannot ascertain after what manner the first true brains were lodged, and what those modifications were which their protecting box, the cranium, received in the subsequent periods. Independently of its own special interest, the inquiry will be found to have a direct bearing on our general subject.
CEREBRAL DEVELOPMENT OF THE EARLIER VERTEBRATA.
ITS APPARENT PRINCIPLE.
It is held by a class of naturalists, some of them of the highest standing, that the skulls of the vertebrata consist, like the columns to which they are attached, of vertebral joints, composed each, in the more typical forms of head, as they are in the trunk, of five parts or elements,—the centrum or body, the two spinous processes which enclose the spinal cord, and the two ribs. These cranial vertebræ, four in number, correspond, it is said, to the four senses that have their seat in the head: there is the nasal vertebra, the centrum of which is the vomer, its spinal processes the nasal and ethmoid bones, and its ribs the upper jaws; there is the ocular vertebra, the centrum of which is the anterior portion of the sphenoid bone, its spinal processes the frontals, and its ribs the under jaws; there is the lingual vertebra, the centrum of which is the posterior sphenoid bone, its spinal processes the parietals, and its ribs the hyoid and branchial bones,—portions of the skeleton largely developed in fishes; and, lastly, there is the auditory vertebra, the centrum of which is the base of the occipital bone, and its spinal processes the occipital crest, and which in the osseous fishes bears attached to it, as its ribs, the bones of the scapular ring. And the cerebral segments thus constructed we find represented in typical diagrams of the skull, as real vertebræ. Professor Owen, in his lately published treatise on “The Nature of Limbs,”—work charged with valuable fact, and instinct with philosophy,—figures in his draught of the archetypal skeleton of the vertebrata, the four vertebræ of the head, in a form as unequivocally such as any of the vertebræ of the neck or body.
Now, for certain purposes of generalization, I doubt not that the conception may have its value. There are in all nature and in all philosophy certain central ideas of general bearing, round which, at distances less or more remote, the subordinate and particular ideas arrange themselves,