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,

In the classifications of the naturalist, for instance, all species range round some central generic idea; all genera round some central idea, to which we give the name of order; all orders round some central idea of class; all classes round some central idea of division; and all divisions round the interior central idea which constitutes a kingdom. Sir Joshua Reynolds forms his theory of beauty on this principle of central ideas. “Every species of the animal, as well as of the vegetable creation,” he remarks, “may be said to have a fixed or determinate form, towards which nature is continually inclining, like various lines terminating in a centre; or it may be compared to pendulums vibrating in different directions over one central point, which they all cross, though only one of their number passes through any other point.” He instances, in illustrating his theory, the Grecian beau ideal of the human nose, as seen in the statues of the Greek deities. It formed a straight line; whereas all deformity of nose is of a convex or concave character, and occasioned by either a rising above or a sinking below this medial line of beauty. And it may be of use, as it is unquestionably of interest, to conceive, after this manner, of a certain type of skeleton, embodying, as it were, the central or primary type of all vertebral skeletons, and consisting of a double range of rings, united by the bodies of the vertebræ, as the two rings of a figure 8 are united at their point of junction; the upper ring forming the enclosure of the brain,—spinal, and cephalic; the lower that of the viscera,—respiratory, circulatory, and digestive. Such is the idea embodied in Professor Owen’s archetypal skeleton. It is a series of vertebræ composing double rings,—their brain-rings comparatively small in the vertebræ of the trunk, but of much greater size in the vertebræ of the head. But it must not be forgotten, that central ideas, however necessary to the classification of the naturalist, are not historic facts. We may safely hold, with the philosophic painter, that the outline of the typical human nose is a straight line; but it would be very unsafe to hold, as a consequence, that the first men had all straight noses. And when we find it urged by at least one eminent assertor of the development hypothesis,—Professor Oken,—that light was the main agent in developing the substance of nerve,—that the nerves, ranged in pairs, in turn developed the vertebræ, each vertebra being but “the periphery or envelope of a pair of nerves,”—and that the nerves of those four senses of smell, sight, taste, and hearing, which, according to the Professor, “make up the head,” originated the four cranial vertebræ which constitute the skull,—it becomes us to test the central idea, thus converted into a sort of historic myth, by the realities of actual history. What, then, let us inquire, is the real history of the cerebral development of the vertebrata, as recorded in the rocks of the earlier geologic periods?

Though the vertebrata existed in the ichthyic form throughout the vastly extended Silurian period, we find in that system no remains of the cranium: the Silurian fishes seem, as has been already said, (page 53,) to have been exclusively Placoid, and the purely cartilaginous box formed by nature for the protection of the brain in this order has in no case been preserved. Teeth, and, in at least one or two instances, the minute jaws over which they were planted have been found, but no portion of the skull. We know, however, that in the fishes of the same order which now exist, the cranium consists of one undivided piece of a cartilaginous substance, set thickly over its outer surface with minute polygonal points of bone, (fig. 7,) composed internally of star-like rays, that radiate from the centre of ossification, and that present, in consequence, seen through a microscope, the appearance of the polygonal cells of a coral of the genus Astrea. The pattern induced is that of stars set within polygons. Along the sides or top of this unbroken cranial box, that exhibits no mark of suture, we find the perforations through which the nerves of smell, sight, taste, and hearing passed from the brain outwards, and see that they have failed to originate distinct vertebral envelopes for themselves;—they all lodge in one undivided mansion-house, and have merely separate doors. We find, further, that the homotypal ribs of the entire cranium consist, not of four, but simply of a single pair, attached to the occiput, and which serves both to suspend the jaws, upper and nether, in their place under the middle of the head, and to lend support to the hyoid and branchial framework; while the scapular ring we find existing, as in the higher vertebrata, not as a cerebral, but as a cervical or dorsal appendage. In the wide range of the animal kingdom there are scarce any two pieces of organization that less resemble one another in form than the vertebræ of the placoids resemble their skulls; and the difference is not merely external, but extends to even their internal construction. In both skull and vertebræ we detect an union of bone and cartilage; but the bone of each vertebra forms an internal continuous nucleus, round which the cartilage is arranged, whereas in the skulls it is the cartilage that is internal, and the bone is spread in granular points over it. If we dip the body of one of the dorsal vertebræ of a herring into melted wax, and then withdraw it, we will find it to represent in its crusted state the vertebral centrum of a Placoid,—soft without, and osseous within; but in order to represent the placoid skull, we would have first to mould it out of one unbroken piece of wax, and then to cover it over with a priming of bone-dust. And such is the effect of this arrangement, that, while the skull of a Placoid, exposed to a red heat, falls into dust, from the circumstance that the supporting framework on which the granular bone was arranged perishes in the fire, the vertebral centrum, whose internal framework is itself bone, and so not perishable, comes out in a state of beautiful entireness,—resembling in the thornback a squat sand-glass, elegantly fenced round by the lateral pillars, (fig. 8, b;) and in the dog-fish (a) a more elongated sand-glass, in which the lateral pillars are wanting. Such are the heads and vertebral joints of the existing Placoids; and such, reasoning from analogy, seem to have been the character and construction of the heads and vertebral joints of the Placoids of the Silurian period,—earliest-born of the Vertebrata.

The most ancient brain-bearing craniums that have come down to us in the fossil state, are those of the Ganoids of the Lower Old Red Sandstone; and in these fishes the true skull appears to have been as entirely a simple cartilaginous box, as that of the Placoids of either the Silurian period or of the present time, or of those existing Ganoids, the sturgeons. In the Lower Old Red genera Cheiracanthus and Diplacanthus, though the heads are frequently preserved as amorphous masses of colored matter, we detect no trace of internal bone, save perhaps in the gill-covers of the first-named genus, which were fringed by from eighteen to twenty minute osseous rays. The cranium seems to have been covered, as in the shark family, by skin, and the skin by minute shagreen-like scales; and all of the interior cerebral framework which appears underneath exists simply as faint impressions of an undivided body, covered by what seem to be osseous points,—the bony molecules, it is probable, which encrusted the cartilage. The jaws, in the better specimens, are also preserved in the same doubtful style, and this state of keeping is the common one in deposits in which every true bone, however delicate, presents an outline as sharp as when it occupied its place in the living animal. The dermal or skin-skeleton of both genera, which consisted, as has been shown (pages 55, 56) of shagreen-like osseous scales and slender spines, both brilliantly enamelled, is preserved entire; where as the interior framework of the head exists as mere point speckled impressions; and the inference appears unavoidable that parts which so invariably differ in their state of keeping now, must have essentially differed in their substance originally.

Now, in the Cheiracanthus we detect the first faint indications of a peculiar arrangement of the dermal skeleton, in relation to certain parts of the skeleton within, which—greatly more developed in some of its contemporaries—led to important results in the general structure of these Ganoids, and furnishes the true key to the character of the early ganoid head. In such of the existing Placoids as I have had an opportunity of examining, the only portions of the dermal skeleton of bone which conform in their arrangement to portions of the interior skeleton of cartilage, are the teeth, which are always laid on a base of skin right over the jaws: there is also an approximation to arrangement of a corresponding kind, though a distant one, in those hook-armed tubercles of certain species of rays which run along the vertebral column; but in the shagreen by which the creatures are covered I have been able to detect no such arrangement. Whether it occurs on the fins, the body, or the head, or in the scale form, or in that of the prickle, it manifests the same careless irregularity. And on the head and body of the Cheiracanthus, and on all its fins save one, the shagreen-like scales, though laid down more symmetrically in lines than true shagreen, manifested an equal absence of arrangement in relation to the framework within. On that one fin, however;—the caudal,—the scales, passing from their ordinary rhomboidal to a more rectangular form, ranged themselves in right lines over the internal rays, (fig. 9, a,) and imparted to these such strength as a splint of wood or whalebone fastened over a fractured toe or finger imparts to the injured digit,—a provision which was probably rendered necessary in the case of this important organ of motion, from the circumstance that it was the only fin which the creature possessed that was not strengthened and protected anteriorly by a strong spine. In the Cheirolepis,—a contemporary fish, characterized, like its cogeners the Cheiracanthus and Diplacanthus, by shagreen-like scales, but in which the spines were wanting,—we find a farther development of the provision. In all the fins the richly-enamelled dermal-covering was arranged in lines over the rays, (fig. 9, b;) and the scale, which assumes in the fins, like the scales on the tail of the Cheiracanthus, though somewhat more irregularly, a rectangular shape, is so considerably elongated, that it assumes for its normal character as a scale, that of the joint of an external ray. A similar arrangement of external protection takes place in this genus over the bones of the head; the cartilaginous jaws receive their osseous dermal covering, and, with these, the hyoid bones, the opercules, and the cranium. And it is in these dermal plates, which covered an interior skull, of which, save in one genus,—the Dipterus,—not a vestige remains in any of the Old Red fishes thus protected, that we first trace what seem to be the homologues of the cranial bones of the osseous fishes,—at least their homologues so far as the cuticular can represent the internal. They appear for the first time, not as modified spinous processes, broadened, as in the carapace of the Chelonians, into osseous plates, but like those corneous external plates of this order of reptiles, (known in one species as the tortoise-shell of commerce,) the origin of which is purely cuticular, and which evince so little correspondence in their divisions with the sutures of the bones on which they rest, that they have been instanced, in their relation to the joinings beneath, as admirable illustrations of the cross-banding of the mechanician.

In the heads of the osseous fishes, the cranium proper, though consisting, like the skulls of birds, reptiles, and mammals, of several bones, exists from snout to nape, and from mastoid to mastoid, as one unbroken box; whereas all the other bones of the head, such as the maxillaries and intermaxillaries, the lower jaws, the opercular appendages, the branchial arches, and the branchiostegous rays, are connected but by muscle and ligament, and fall apart under the putrefactive influences, or in the process of boiling. This unbroken box, which consists, in the cod, of twenty-five bones, is the homologue of that cranial box of the Placoids which consists of one entire piece, and the homotype, according to Oken, of the bodies and spinal processes of four vertebræ; while the looser bones which drop away represent their ribs. The upper surface of the box,—that extending from the nasal bone to the nape,—is the only part over which a dermal buckler could be laid, as it is the only part with which the external skin comes in contact; and so it is between this upper surface and the cranial bucklers of the earlier Ganoids that we have to institute comparisons. For it is a curious fact, that, with the exception of the Old Red genera Acanthodus, Cheiracanthus, and Diplacanthus,[14] all the Ganoids of the period in which Ganoids first appear have dermal bucklers placed right over their true skulls, and that these, though as united in their parts as the bones proper to the cranium in quadrupeds and fishes, are composed of several pieces, furnished each with its independent centre of ossification. The Dipterians, the Cœlacanths, the Cephalaspians, and at least one genus placed rather doubtfully among the Acanths,—the genus Cheirolepis,—all possessed cranial bucklers extending from the nape to the snout, in which the plates, various, in the several genera, in form and position, were fast soldered together, though in every instance the lines of suture were distinctly marked.

On each side of this external cranium the various cerebral plates, like the corresponding cerebral ribs in the osseous fishes, were free, at least not anchylosed together; and some of their number unequivocally performed, in part at least, the functions of two of these cerebral ribs, viz. the upper and under jaws, with the functions of the opercular appendages attached to the latter. In the cod, as in most other osseous fishes, the upper portion of the cranium consists of thirteen bones, which represent, however, only seven bones in the human skull,—the nasal, the frontal, the two parietal, the occipital, and one-half the two temporal bones. And whereas in man, and in most of the mammals, there are four of these placed in the medial line,—the four which, according to the assertors of the vertebral theory, form the spinal crests of the four cerebral vertebræ,—in the cod there are but three. The super-occipital bone, A, (fig. 10,) pieces on to the superior frontal, C, C, C; and the parietals, B, B, which in the human subject from the upper and middle portions of the cranial vault, are thrust out laterally and posteriorly, and take their places, in a subordinate capacity, on each side of the super-occipital. This is not an invariable arrangement among fishes;—in the carp genus, for instance, the parietals assume their proper medial place between the occipital and frontal bones; but so very general is the displacement, that Professor Owen regards it as characteristic of the great ichthyic class, and as the first example in the vertebrata, reckoning from the lower forms upwards, of a sort of natural dislocation among the bones,—“a modification,” he remarks, “which, sometimes accompanied by great change of place, has tended most to obscure the essential nature of parts, and their true relations to the archetype.”

Of all the cerebral bucklers of the first ganoid period, that which best bears comparison with the cranial front of the cod is the buckler of the Coccosteus, (fig. 11.) The general proportions of this portion of the ancient Cephalaspian head differ very considerably from those of the corresponding part in the modern cycloid one; but in their larger divisions, the modern and the ancient answer bone to bone. Three osseous plates in the Coccosteus, A, C, I, the homologues, apparently, of the occipital, frontal, and nasal bones, range along the medial line. The apparent homologues of the parietals, B, B, occupy the same position of lateral displacement as the parietals of the cod and of so many other fishes. The posterior frontals, F, and the anterior frontals, D, also occupy places relatively the same, though the latter, which are of greater proportional size, encroach much further, laterally and posteriorly, on the superior frontal C, C, C, and sweep entirely round the upper half of the eye orbits, 2, 2. The apparent homologue of the mastoid bone, E, which also occupies its proper place, joins posteriorly to a little plate, a, imperfectly separated in most specimens from the parietal, but which seems to represent the par-occipital bone; and it is a curious circumstance, that as, in many of the osseous fishes, it is to these bones that the forks of the scapular arch are attached, they unite in the Coccosteus in furnishing, in like manner, a point of attachment to the cuirass which covered the upper part of the creature’s body. Of the true internal skull of the Coccosteus there remains not a vestige Like that of the sturgeon, it must have been a perishable cartilaginous box.

In the Osteolepis,—an animal the whole of whose external head I have, at an expense of some labor, and from the examination of many specimens, been enabled to restore,—the cranial buckler (fig. 12) was divided in a more arbitrary style; and we find that an element of uncertainty mingles with our inferences regarding it, from the circumstance that some of its lines of division, especially in the frontal half, were not real sutures, but formed merely a kind of surface-tatooing, resorted to as if for purposes of ornament. The cranial buckler of the Asterolepis exhibited, as I shall afterwards have occasion to show, a similar peculiarity;—both had their pseudo-sutures, resembling those false joints introduced by the architect into his rusticated basements, in order to impart the necessary aspect of regularity to what is technically termed the coursing and banding of the fabric. We can however, determine, notwithstanding the induced obscurity that the buckler of the Osteolepis was divided transversely in the middle into two main parts or segments,—an occipital part, C, and a frontal part, A; and that the occipital segment seems to include also the parietal and mastoid plates, and the frontal segment to comprise, with its own proper plates, not only the nasal plate, but also the representative of the anterior part of the vomer. All, however, is obscure. But in our uncertainty regarding the homologies of the divisions of this dermal buckler, let us not forget the homology of the buckler itself, as a whole, with the upper surface of the true cranium in the osseous fishes. Though frequently crushed and broken, it exists in all the finer specimens of my collection as a symmetrically arranged collocation of enamelled plates, as firmly united into one piece, though they all indicate their distinct centres of ossification, as the corresponding surface of the cranium in the carp or cod. The lateral curves in the frontal part immediately opposite the lozenge-shaped plate in the centre, show the position of the eyes, which were placed in this genus, as in some of the carnivorous turtles, immediately over the mouth,—an arrangement common to almost all the Ganoids of the Lower Old Red Sandstone. The nearly semicircular termination of the buckler formed the creature’s snout; and in the Osteolepis, as in the Glyptolepis and the Diplopterus, it was armed on the under side, like the vomer of so many of the osseous fishes, with sharp teeth. Some of my specimens indicate the nasal openings a little in advance of the eyes. The nape of the creature was covered by three detached plates, (9, 9, 9, fig. 13,) which rested upon anterior dorsal scales, and whose homologies, in the osseous fishes, may possibly be found in those bones which, uniting the shoulder-bones to the head, complete the scapular belt or ring. The operculum we find represented by a single plate (8) which had attached to it, as its sub-operculum, a plate (13) of nearly equal size, (see figs. 14 and 15.) Four small plates (2, 4, 5) formed the under curve of the eyes, described in many of the osseous fishes by a chain of small bones or ossicles; a considerably larger plate (6) occupied the place of the preopercular bone; while the intermaxillaries had their representatives in well-marked plates, (3, 3,) which, in the genera Osteolepis, Diplopterus, and Glyptolepis, we find bristling so thickly with teeth along their lower edges, as to remind us of the miniature saws employed by the joiner in cutting out circular holes. These external intermaxillaries did not, as in the perch or cod, meet in front of the nasal bone and vomer, but joined on at the side, a little in advance of the eyes, leaving the rounded termination of the cranial buckler, which, like the intermaxillaries, was thickly fringed with teeth, to form, as has been already said, the creature’s snout.

The under jaws (10)—strongly-marked bones in at least all the Dipterian and Cœlacanth genera—we find represented externally by massy plates, bearing, like those of the upper jaw, their range of teeth. As shown in a well-preserved specimen of the lower jaw of Holoptychius, in my possession, they were boxes of bone enclosing a bulky nucleus of cartilage, which, in approaching towards the condyloid process, where great strength was necessary, was thickly traversed by osseous cancelli, and passed at the joint into true bone. It is in the under jaws of the earlier Ganoids that we first detect a true union of the external with the internal skeleton,—of the bony plates and teeth, which were mere plates and teeth of the skin, with the osseous, granular walls which enclosed at least all the larger pieces of the cartilaginous framework of the interior. The jaws of the Rays and Sharks, formed of cartilage, and fenced round on their sides and edges by their thin coverings of polygonal, bony points, are wholly internal and skin-covered; whereas the teeth, which rest on the soft cuticular integument right over them, are as purely dermal as the surrounding shagreen. Teeth and shagreen may, we find, be alike stripped off with the skin. Now, in the earlier ganoidal jaw, two sides of the osseous box which it composed,—its outer and under sides,—were mere dermal plates, representative of the skin of the placoids, or of their shagreen; while the other two,—its upper and inner sides,—seem to have been developments of the interior osseous walls which covered the endo-skeletal cartilage. Nor is it unworthy of notice, that the reptile fishes of the period had their ichthyic teeth ranged along the edge of an exterior dermal plate which covered the outer side of the jaw; whereas their reptile teeth were planted on a plate, apparently of interior development, which covered its upper edge. It is further worthy of remark, that while the teeth of the dermal plate,—themselves also dermal,—seem as if they had grown out of it, and formed part of it,—just as the teeth of the Placoids grew out of the skin on which they rest,—the reptile teeth within rested in shallow pits,—the first faint indications of true sockets.

That space included within the arch formed by the sweep of the under jaws, which we find occupied in the osseous fishes by the hyoid bones and the branchiostegous rays, was filled up externally, in the Dipterians and Cœlacanths, and in at least two genera of Cephalaspians, by dermal plates; in some genera, such as the Diplopterus, by three plates; in others, such as the Holoptychius and Glyptolepis, by two; and in the Asterolepis, as we shall afterwards see, by but a single plate. In the Osteolepis these plates were increased to five in number, by the little plates 14, 14, (fig. 14,) which, however, may have been also present in the Diplopterus, though my specimens fail to show them. The general arrangement was of much elegance,—an elegance, however, which, in the accompanying restorations, the dislocation of the free plates, drawn apart to indicate their detached character, somewhat tends to obscure. But the position of the eyes must have imparted to the animal a sinister reptile-like aspect. The profile, (fig. 15,) the result, not of a chance-drawn outline, arbitrarily filled up, but produced by the careful arrangement in their proper places of actually existing plates, serves to show how perfectly the dermo-skeletal parts of the creature were developed. Some of the animals with which we are best acquainted, if represented by but their cuticular skeleton, would appear simply as sets of hoofs and horns. Even the tortoise or pengolin would present about the head and limbs their gaps and missing portions; but the dermo-skeleton of the Osteolepis, composed of solid bone, and burnished with enamel, exhibited the outline of the fish entire, and, with the exception of the eye, the filling up of all its external parts. Presenting outside, in its original state, no fragment of skin or membrane, and with even its most flexible organs sheathed in enamelled bone, the Osteolepis must have very much resembled a fish carved in ivory; and, though so effectually covered, it would have appeared, from the circumstance, that it wore almost all its bone outside, as naked as the human teeth.

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.

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.

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.

The protection afforded by the osseous covering of both the upper and under surface of the cranium of this ichthyolite has resulted, in several instances, in the preservation, though always in a greatly compressed state, of the cranium itself, and the consequent exhibition of two very important cranial cavities, the brain-pan proper, and the passage through which the spinal cord passed into the brain. In the sturgeon the brain occupies nearly the middle of the head; and there is a considerable part of the occipital region traversed by the spine in a curved channel, which, seen in profile, appears wide at the nape, but considerably narrower where it enters the brain-pan, and altogether very much resembling the interior of a miniature hunting-horn. And such exactly was the arrangement of the greater cavities in the head of the Dipterus. The portion of the cranium which was overlaid by what may be regarded as the occipital plate was traversed by a cavity shaped like a Lilliputian bugle-horn; while the hollow in which the brain was lodged lay under the two parietal plates, and the little elliptical plate in the centre. The accompanying print, (fig. 22,) though of but slight show, may be regarded by the reader with some little interest, as a not inadequate representation of the most ancient brain-pan on which human eye has yet looked,—as, in short, the type of cell in which, myriads of ages ago, in at least one genus, that mysterious substance was lodged, on whose place and development so very much in the scheme of creation was destined to depend. The specimen from which the figure is taken was laid open laterally by chance exposure to the waves on the shores of Thurso, another specimen, cut longitudinally by the saw of the lapidary, yields a similar section, but greatly more compressed in the cavities; on which, of course, as unsupported hollows, the compression to which the entire cranium had been exposed chiefly acted. When the top and bottom of a box are violently forced together, it is the empty space which the box encloses that is annihilated in consequence of the violence.

It is deserving of notice, that the analogies of the cranial cavities in this ancient Ganoid should point so directly on the cranial cavities of that special Ganoid of the present time which unites a true skull of cartilage to a dermal skull of osseous plates,—a circumstance strongly corroborative of the general evidence, negative and positive, on which I have concluded that the true skulls of the first Ganoids were also cartilaginous. It is further worthy of observation, that in all the sections of the cranium of Dipterus which I have yet examined, the internal line is continuous, as in the Placoids, from nape to snout, and that the true skull presents no trace of those cerebral vertebræ of which skulls are regarded by Oken and his disciples as developments. Historically at least, the progress of the ichthyic head seems to have been a progress from simple cartilaginous boxes to cartilaginous boxes covered with osseous plates, that performed the functions whether active or passive, of internal bones; and then from external plates to the interior bones which the plates had previously represented, and whose proper work they had done.

The principle which rendered it necessary that the divisions which exist in the dermal skulls of the first Ganoids should so closely correspond with the divisions which exist in the internal skulls of the osseous fishes of a greatly later period, does not seem to lie far from the surface. Of the solid parts of the ichthyic head, a certain set of pieces afford protection to the brain and cerebral nerves, and to some of the organs of the senses, such as those of seeing and hearing; while another certain set of pieces constitute the framework through which an important class of functions, manducatory and respiratory, are performed. The protective bones of merely passive function are fixed, whereas the bones of active function, such as the jaws, the osseous framework of the opercules, and the hyoid bones, are to the necessary extent free, i. e. capable of independent motion. Of course, the detached character necessary to the free cerebral bones would be equally necessary in cerebral plates united dermally to the pieces of the cartilaginous framework, which performed in the ancient fish the functions of these free bones. And hence jaw plates, opercular plates, and hyoid plates, whose homological relation with recent jaws and opercular and hyoid bones cannot be mistaken. They were operative in performing identical mechanical functions, and had to exist, in consequence, in identical mechanical conditions. And an equally simple, though somewhat different principle, seems to have regulated the divisions of the fixed cranial bucklers of the Old Red Ganoids, and to have determined their homologies with the fixed cerebral bones of the osseous fishes.

These cranial bucklers, extending from nape to snout, protected the exposed upper surface of the cartilaginous skull, and conformed to it in shape, as a helmet conforms to the shape of the head, or a breast-plate to the shape of the chest. And as the cartilaginous heads resembled in general outline the osseous ones, the buckler which covered their upper surface resembled in general outline the upper surface of the osseous skull. It was in no case entirely a flat plate; but in every species rounded over the snout and in most species at the sides; and so, in order that its characteristic proportions might be preserved throughout the various stages of growth in the head which it covered, it had to be formed from several distinct centres of ossification, and to extend in area around the edges of the plates originated from these. The workman finds no difficulty in adding to the size of a piece of straight wall, whether by heightening or lengthening it; but he cannot add to the size of a dome or arch, without first taking it down, and then erecting it anew on a larger scale. In the domes and arches of the animal kingdom, the problem is solved by building them up of distinct pieces, few or many, according to the demands of the figure which they compose, and rendering these pieces capable of increase along their edges. It is on this principle that the Cystidea, the Echinidæ, the Chelonian carapace and plastron, and the skulls of the osseous Vertebrata, are constructed. It is also the principle on which the cranial bucklers of the ancient Ganoids were formed.[17] And from the general resemblance in figure of these bucklers to the upper surface of the osseous skull, the separate parts necessary for the building up of the one were anticipated, by many ages, in the building up of the other; just as we find external arches of stone which were erected two thousand years ago, constructed on the same principle, and relatively of the same parts, as internal arches of brick built in the present age. Doubtless, however, with this mechanical necessity for correspondence of parts in the formation of corresponding erections, there may have mingled that regard for typical resemblance which seems so marked a characteristic of the style, if I may so express myself, in which the Divine Architect gives expression to his ideas. The external osseous buckler He divided after the general pattern which was to be exemplified, in latter times, in the divisions of the internal osseous skull; as if in illustration of that “ideal exemplar” which dwelt in his mind from eternity, and on the palpable existence of which sober science has based deductions identical in their scope and bearing with some of the sublimest doctrines of the theologian. “The recognition,” says Professor Owen, “of an ideal exemplar for the vertebrated animals, proves that the knowledge of such a being as man existed before man appeared; for the Divine mind which planned the archetype also foreknew all its modifications. The archetypal idea was manifested in the flesh, under divers such modifications, upon this planet, long prior to the existence of those animal species that actually exemplify it.”

But while we find place in that geological history in which every character is an organism, for the “ideal exemplar” of Professor Owen, we find no place in it for the vertebræ-developed skull of Professor Oken. The true genealogy of the head runs in an entirely different line. The nerves of the cerebral senses did not, we find, originate cerebral vertebræ, seeing that the heads of the first and second geologic periods had their cerebral nerves, but not their cerebral vertebræ; and that what are regarded as cerebral-vertebræ appear for the first time, not in the early fishes, but in the reptiles of the Coal formation. The line of succession through the fish, indicated by the Continental assertor of the development hypothesis, is a line cut off. All the existing evidence conspires to show that the placoid heads of the Silurian system were, like the placoid heads of the recent period, mere cartilaginous boxes; and that in the succeeding system there existed ganoidal heads, that to the internal cartilaginous box added external plates of bone, the homologues, apparently,—so far at least as the merely cuticular could be representative of the endo-skeletal,—of the opercular, maxillary, frontal, and occipital bones in the osseous fishes of a long posterior period,—fishes that were not ushered upon the scene until after the appearance of the reptile in its highest forms and of even the marsupial quadruped.