Part 10

The inner surface of the cranial buckler of _Asterolepis_, (fig. 28,)—that which rested on the cartilaginous box which formed the creature’s interior skull,—stands out in bolder relief from the stone than its outer surface, and forms a more picturesque object. Like the inner surfaces of the bucklers of _Coccosteus_ and _Pterichthys_, but much more thickly than these, it was traversed by minute channelled markings, somewhat resembling those striæ which may be detected in the flatter bones of the ordinary fishes, and which seem in these to be mere interstices between the osseous fibres. And in the plates, as in the bones, they radiate from the centres of ossification, which are comparatively dense and massy, towards the thinner overlapping edges. These radiating lines are equally well marked in the cerebral bones of the human fœtus. The three converging ridges on the outer surface we find on the inner surface also,—the lateral ones a little bent in the middle, but so directly opposite those outside, that the thickening of the buckler which takes place along their line is at least as much a consequence of their inner as of their outer elevation over the general platform. A fourth bar ran transversely along the nape, and formed the cross beam on which the others rested; for the three longitudinal ridges may be properly regarded as three strong beams, which, extending from the transverse beam at the nape to the front, where they converged like the spokes of a wheel at the nave, gave to the cranial roof a degree of support of which, from its great flatness, it may have stood in need. In cranial bucklers in which the average thickness of the plates does not exceed three _eighth_ parts of an inch, their thickness in the centre of the ridges exceeds three _quarters_. The head of the largest crocodile of the existing period is defended by an armature greatly less strong than that worn by the _Asterolepis_ of the Lower Old Red Sandstone. Why this ancient Ganoid should have been so ponderously helmed we can but doubtfully guess; we only know, that when nature arms her soldiery, there are assailants to be resisted and a state of war to be maintained. The posterior central plate, the homologue apparently of the occipital bone, was curiously carved into an ornate massive leaf, like one of the larger leaves of a Corinthian capital, and terminated beneath, where the stem should have been, in a strong osseous knob, fashioned like a pike head. Two plates immediately over it, the homologues of the superior frontal bone, with the little nasal plate which, perched atop in the middle, lay between the creature’s eyes, resembled the head and breast in the female figure, at least not less closely than those of the “lady in the lobster;” the posterior frontal plates in which the outer and nether half of the eye orbits were hollowed formed a pair of sweeping wings, and thus in the centre of the buckler we are presented with the figure of an angel, robed and winged, and of which the large sculptured leaf forms the body, traced in a style in no degree more rude than we might expect to see exemplified on the lichen-encrusted shield of some ancient tombstone of that House of Avenel which bore as its arms the effigies of the Spectre Lady. Children have a peculiar knack in detecting such resemblances; and the discovery of the angel in the cranium of the _Asterolepis_ I owe to one of mine.

It is on this inner side of the cranial buckler, where there are no such pseudo-joinings indicated as on the external surface, that the homologies of the plates of which it is composed can be best traced. It might be well, however, ere setting one’s self to the work of comparison, to examine the skulls of a few of the osseous fishes of our coast, and to mark how very considerably they differ from one another in their lines of suture and their general form. The cerebral divisions of the conger-eel, for instance, are very unlike those of the haddock or whiting; and the sutures in the head of the gurnard are dissimilarly arranged from those in the head of the perch. And after tracing the general type in the more anomalous forms, and finding, with Cuvier, that in even these the “skull consists of the same bones, though much subdivided, as the skulls of the other vertebrata,” we will be the better qualified for grappling with the not greater anomalies which occur in the cranial buckler of the _Asterolepis_. The occipital plate, _A_, _a_, _a_, (fig. 29,) occupies its ordinary place opposite the centre of the nape; the two parietals, B, B, rest beside it in their usual ichthyic position of displacement; the superior frontal we find existing, as in the young of many animals, in two pieces, C, C; the nasal plate I, placed immediately in advance of it, is flanked, as in the cod, by the anterior frontals, D, D; the posterior frontals, F, F, which, when viewed as in the print, from beneath, seem of considerable size, and describe laterally and posteriorly about one half the eye orbits, have their area on the exterior surface greatly reduced by the overriding squamose sutures of the plates to which they join; and lastly, two of these overlying plates, E, E,—which, occurring in the line of the lateral bar or beam, are of great strength and thickness, and lie for two thirds of their length along the parietals, and for the remaining third along the superior frontals,—represent the mastoid bones. Such, so far as I have been yet able to read the cranial buckler of the _Asterolepis_, seem to be the homologies of its component plates.

There were no parts of the animal more remarkable than its jaws. The under jaws,—for the nether maxillary consisted, in this fish, as in the placoid fishes, and in the quadrupeds generally, of two pieces joined in the middle,—were, like those of the _Holoptychius_, boxes of bone, which enclosed central masses of cartilage. The outer and under sides were thickly covered with the characteristic star-like tubercles; and along the upper margin or lip there ran a thickly-set row of small broadly-based teeth, planted as directly on the edge of the exterior plate as iron spikes on the upper edge of a gate (fig. 30.) Mr. Parkinson expresses some wonder, in his work on fossils, that, in a fine ichthyolite in the British Museum, not only the _teeth_ should have been preserved, but also the _lips_; but we now know enough of the construction of the ancient Ganoids to cease wondering. The _lips_ were formed of as solid bone as the teeth themselves, and had as fair a chance of being preserved entire; just as the metallic rim of a _cogged_ wheel has as fair a chance of being preserved as the metallic cogs that project from it. Immediately behind the front row,—in which the teeth present the ordinary ichthyic appearance,—there ran a thinly-set row of huge _reptile_ teeth, based on an interior platform of bone, which formed the top of the cartilage-enclosing box composing the jaw. These were at once bent outwards and twisted laterally, somewhat like nails that have been drawn out of wood by the claw of a carpenter’s hammer, and bent awry with the wrench, (fig. 31.) They were furrowed longitudinally from point to base by minute thickly-set striæ and were furnished laterally, in most of the specimens though not in all, with two sharp cutting edges. The reptile had as yet no existence in creation; but we see its future coming symbolized in the dentition of this ancient Ganoid: it, as it were, shows us the _crocodile_ lying entrenched behind the fish. The interior structure of these reptile teeth is very remarkable. In the longitudinal section we find numerous cancelli, ranged lengthwise along the outer edges, but much crossed, net-like, within,—greatly more open towards the base than at the point,—and giving place in the centre to a hollow space, occasionally traversed by a few slim osseous partitions. In the transverse section these cancelli are found to radiate from the open centre towards the circumference, like the spokes of a wheel from the nave; and each spoke seems as if, like Aaron’s rod, it had become instinct with vegetative life, and had sprouted into branch and blossom. Seen in a microscope of limited field, that takes in, as in the accompanying print, (fig. 32,) not more than a fourth part of the section, the appearance presented is that of a well-trained wall tree. And hence the generic name _Dendrodus_, given by Professor Owen to teeth found detached in the deposits of Moray, when the creatures to which they had belonged were still unknown,—a name, however, which will, I suspect, be found synonymous rather with that of a family than of a genus; for so far as I have yet examined, I find that the dendrodic or tree-like tooth, was in at least the Old Red Sandstone, a characteristic of all the Cœlacanth family. I may mention, however, as a curious subject of inquiry, that the Cœlacanths of the Coal Measures seem to have had their reptile teeth formed of pure ivory,—a substance, which I have not yet detected among the reptile-fish of the Old Red. Towards the base of the reptile teeth of _Asterolepis_, the interstices between the branches greatly widen, as in the branches of a tree in winter divested of its foliage, (fig. 33, _c_;) the texture also opens towards the base in the _fish_-teeth, outside, in which, however, the pattern in the transverse section is greatly less complex and ornate than that which the reptile teeth exhibits. When cut across near the point, they appear each as a thick ring, (_b_,) traversed by lines that radiate towards the centre; when cut across about half way down, they somewhat resemble, seen under a high magnifying power those cast-iron wheels on which the engineer mounts his railway carriages, (_a_.) In the longitudinal section their line of junction with the jaw is marked by numerous openings, but by no line of division, and they appear as thickly dotted by what were once canaliculi, or life points, as any portion of the dermal bone on which they rest.

It seems truly wonderful, when one considers it, to what minute and obscure ramifications that variety of pattern which nature so loves to maintain is found to descend. It descends in the fishes, both recent and extinct, to even the microscopic structure of their teeth; and we find, in consequence, not less variety of figure in the sliced fragments of the teeth of the ichthyolites of a single formation, than in the carved blocks of an extensive calico print-yard. Each _species_ has its own distinct pattern, as if, in all the individuals of which it consisted, the same block had been employed to stamp it; and each _genus_ its own general _type_ of pattern, as if the same radical idea, variously altered and modified, had been wrought upon in all. In the _Dendrodic_ (Cœlacanth?) family, for instance, it is the radical type, that from a central nave there should radiate, spoke-like, a number of arborescent branches; but in the several genera and species of the family, the branches belong, if I may so express myself, to different shrubs, and present dissimilar outlines. It has appeared to me, that at least a _presumption_ against the transmutation of species might be based on those inherent peculiarities of structure which are thus found to pervade the entire texture of the framework of animals. If we find erections differing from one another merely in external form, we have no difficulty in conceiving how, by additions and alterations, they might be brought to exhibit a perfect uniformity of plan and aspect: _transmutation_,—_development_,—_progression_,—(if one may use such terms,)—seem possible in such circumstances. But if the buildings differ from each other, not only in external form, but also in every brick and beam, bolt and nail, no mere scheme of external alteration could ever induce a real resemblance. Every brick would have to be taken down, and every beam and bolt removed. The problem could not be wrought by the remodelling of an old house: the only mode of solving it would be by the erection of a new one.

Of the upper maxillary bones of the _Asterolepis_, I only know that a considerable fragment of one of the pieces, recognized as such by Agassiz, has been found in the neighborhood of Thurso by Mr. Dick, unaccompanied, however, by any evidence respecting its place or function. It exhibits none of the characteristic tubercles of the dermal bones, and no appearance of teeth; but is simply a long bent bone, resembling somewhat less than the half of an ancient bow of steel or horn,—such a bow as that which Ulysses bended in the presence of the suitors. By some of the Russian geologists this bone was at first regarded as a portion of the arm or wing of some gigantic _Pterichthys_. In the accompanying print (fig. 34) I have borrowed the general outline from that of a specimen of Professor Asmus, of which a cast may be seen in the British Museum; while the shaded portion represents the fragment found by Mr. Dick. The intermaxillary bones, like the dermal plates of the lower jaw, were studded by star-like tubercles, and bristled thickly along their lower edges with the ichthyic teeth, flanked by teeth of the reptilian character. The opercules of the animal consisted, as in the sturgeon, of single plates (fig. 35) of great massiveness and size, thickly tubercled outside, without trace of joint or suture, and marked on their under surface by channelled lines, that radiate, as in the other plates, from the centre of ossification. That space along the nape which intervened between the opercules, was occupied, as in the _Dipterus_ and _Diplopterus_, by three plates, which covered rather the anterior portion of the body than the posterior portion of the head, and which, in the restoration of _Osteolepis_, (fig. 13,) appear as the plates, 9, 9, 9. I can say scarce any thing regarding the lateral plates which lay between the intermaxillaries and the cranial buckler, and which exist in the _Osteolepis_, fig. 13, as the plates 2, 4, 5, 6, and 7; nor do I know how the snout terminated, save that in a very imperfect specimen it exhibits, as in the _Diplopterus_ and _Osteolepis_, a rounded outline, and was set with teeth.

That space comprised within the arch of the lower jaws, in which the hyoid bone and branchiostegous rays of the osseous fishes occur, was filled by a single plate of great size and strength, and of singular form, (fig. 36;) and to this plate, existing as a steep ridge running along the centre of the interior surface, and thickening into a massy knob at the anterior termination, that nail-shaped organism, which I have described as one of the most characteristic bones of the _Asterolepis_, belonged. In the _Osteolepis_, the space corresponding to that occupied by this hyoid plate was filled, as shown in fig. 14, by five plates of not inelegant form; and the divisions of the arch resembled those of a small Gothic window, in which the single central mullion parts into two branches atop. In the _Holoptychius_ and _Glyptolepis_ there were but two plates; for the central mullion, _i. e._ line of division, did not branch atop; and in the _Asterolepis_, where there was no line of division, the strong nail-like bone occupied the place of the central mullion. The hyoidal armature of the latter fish was strongest in the line in which the others were weakest. Each of the five hyoid plates of the _Osteolepis_, or of the two plates of the _Glyptolepis_ or _Holoptychius_, had its own centre of ossification; and in the single plate of _Asterolepis_, the centre of ossification, as shown by the radiations of the fibre, was the nail-head. This head, placed in immediate contact with the strong boxes of bone which composed the under jaw, just where their central joining occurred, seems to have lent them a considerable degree of support, which at such a juncture may have been not unnecessary. In some of the nail-heads, belonging, it is probable, to a different species of _Asterolepis_ from that in which the nail figured in page 7, and the plate in the opposite page, occurred,—for its general form is different, (fig. 37,)—there appear well-marked ligamentary impressions closely resembling that little spongy pit in the head of the human thigh-bone to which what is termed the round ligament is attached. The entire hyoid-plate, viewed on its outer side, resembles in form the hyoid-bone,—or cartilage rather,—of the spotted dog-fish, (_Scyllium stellare_;) but its area was at least a hundred times more extensive than in the largest _Scyllium_, and, like all the dermal plates of the _Asterolepis_, it was thickly fretted by the characteristic tubercles. In the Ray, as in the Sharks, the piece of thin cartilage of which this plate seems the homologue, is a flat, semi-transparent disk; and there is no part of the animal in which the progress of those bony molecules which encrust the internal framework may be more distinctly traced, as if in the act of creeping over what they cover, in slim threads or shooting points,—and much resembling new ice creeping in a frosty evening over the surface of a pool.

That suite of shoulder-bones that in the osseous fishes forms the belt or frame on which the opercules rest, and furnishes the base of the pectorals, was represented in the _Asterolepis_, as in the sturgeon, by a ring of strong osseous plates, which, in one of the two species of which trace is to be found among the rocks of Thurso, were curiously fretted on their external surfaces, and in the other species comparatively smooth. The largest, or coracoidian plate of the ring, as it occurs in the more ornate species, (fig. 38,) might be readily enough mistaken, when seen with only its surface exposed for the ichthyodorulite of some large fish, allied, mayhap, to the _Gyracanthus formosus_ of the Coal Measures; but when detached from the stone, the hollow form and peculiar striæ of the inferior surface serve to establish its true character as a dermal plate. The diagonal furrowings which traversed it, as the twisted flutings traverse a Gothic column moulded after the type of the Apprentice Pillar in Roslin chapel, seem to have underlaid the edge of the opercule; at least I find a similar arrangement in the shoulder-plates of a large species of _Diplopterus_, which are deeply grooved and furrowed where the opercule rested, as if with the design of keeping up a communication between the branchiæ and the external element, even when the gill-cover was pressed closely down upon them. And,—as in these shoulder-plates of the _Diplopterus_ the furrows yield their place beyond the edge of the opercule to the punctulated enamel common to the outer surface of all the creature’s external plates and scales,—we find them yielding their place, in the shoulder-plates of the _Asterolepis_, to the starred tubercles.

A few detached bones, that bear on their outer surfaces the dermal markings, must have belonged to that angular-shaped portion of the head which intervened between the cranial buckler and the intermaxillary bone; but the key for assigning to them their proper place is still to find; and I suspect that no amount of skill on the part of the comparative anatomist will ever qualify him to complete the work of restoration without it. I have submitted to the reader the cranial bucklers of _five_ several genera of the ganoids of the Old Red Sandstone; but no amount of study bestowed on these would enable even the most skilful ichthyologist to restore a _sixth_; nor is the lateral area of the head, which was, I find, variously occupied in each genus, less difficult to restore than the buckler which surmounted it. Two of the more entire of these dermal bones I have figured (fig. 39, _a_ and _b_) in the hope of assisting future inquirers, who, were they to pick up all the other plates, might yet be unable, lacking the figured ones, to complete the whole. The curiously-shaped plate _a_, represented in its various sides by the figures 1, 2, 3, is of an acutely angular form in the transverse section, (the external surface, 1, forming an angle which varies from thirty to forty-five degrees with the base, 3;) and as it lay, it is probable when in its original place, immediately under the edge of the cranial buckler, it may have served to commence the line of deflection from the flat top of the head to the steep descent of the sides, just as what are technically termed the _spur_-stones in a gable-head serve to commence the line of deflection from the vertical outline of the wall to the inclined line of the roof, or as the spring-stones of an arch serve to commence the curve. A few internal bones in my possession are curious, but exceedingly puzzling. The bone _a_, fig. 40, which resembles a rib, or branchiostegous ray, of one of the ordinary fishes, formed apparently part of that osseous _style_ which in fishes such as the haddock and cod we find attached to the suite of shoulder-bones, and which, according to Cuvier, is the analogue of the coracoidian bone, and, according to Professor Owen, the analogue of the clavicle. Fig. _b_ is a mere fragment, broken at both ends, but exhibiting, in a state of good keeping, lateral expansions, like those of an ancient halbert. Fig. _c_, 41, which is also a fragment, though a more considerable one, bears in its thicker and straighter edge a groove like that of an ichthyodorulite, which, however, the bone itself in no degree resembles. Fig. _d_ is a flat bone, of a type common in the skeleton of fishes, but which, in mammals, we find exemplified in but the scapulars. It seems, like these, to have furnished the base to which some suite of movable bones was articulated,—in all likelihood that proportion of the carnal bonelets of the pectoral fins which are attached in the osseous fishes to its apparent homologue, the radius. Fig. _e_, a slim light bone, which narrows and thickens in the centre, and flattens and broadens at each end, was probably a scapula or shoulder-blade,—a bone which in most fishes _splices_ on, as a sailor would say, by squamose jointings, to the coracoidian bone at the one end, and the super-scapular bone at the other. As indicated by its size, it must have belonged to a small individual: it is, however, twice as long, and about six times as bulky, as the scapula of a large cod.