Part 10

With the reader, if he has accompanied me thus far, I shall now pass on to the consideration of the remains of the Asterolepis. Our preliminary acquaintance with the cerebral peculiarities of a few of its less gigantic contemporaries will be found of use in enabling us to determine regarding a class of somewhat resembling peculiarities which characterized this hugest Ganoid of the Old Red Sandstone.

Dermal tubercles of Asterolepis

(Mag. two diameters.)]

The head of the Asterolepis, like the heads of all the other Cœlacanths, and of all the Dipterians, was covered with osseous plates,—its body with osseous scales; and, as I have already had occasion to mention, it is from the star-like tubercles by which the cerebral plates were fretted that M. Eichwald bestowed on the creature its generic name. Agassiz has even erected species on certain varieties in the pattern of the stars, as exhibited on detached fragments; but I am far from being satisfied that we are to seek in their peculiarities of style the characters by which the several species were distinguished. The stellar form of the tubercle seems to have been its normal or most perfect form as it was also, with certain modifications, that of the tubercle of the Coccosteus and Pterichthys; but its development as a complete star was comparatively rare: in most cases the tubercles existed without the rays,—frequently in the insulated pap-like shape, but not rarely confluent, or of an elongated or bent form; and when to these the characteristic rays were added, the stars produced were of a rather eccentric order,—stars somewhat resembling the shadows of stars seen in water. Individual specimens have already been found, on which, if we recognize the form of the tubercle as a specific character, several species might be erected. The accompanying wood-cut (fig. 24) represents, from a Thurso specimen, what seems to be the true normal pattern of these cerebral carvings. Seen in profile (b) the tubercles resemble little hillocks, perforated at their base by single lines of thickly-set caves; while seen from above, (a,) the narrow piers of bone by which the caves are divided take the form of rays. The reader will scarce fail to recognise in this print the coral Monticularia of Lamarck, or to detect, in at least the profile, the peculiarity which suggested the name.

SCALES OF ASTEROLEPIS.

(Nat. size.)

a. Inner surface of scale.

b. Exterior surface.]

PORTION OF CARVED SURFACE OF SCALE.

(Mag. four diameters.)]

The scales which covered the creature’s body (fig. 25) were, in proportion to its size, considerably smaller and thinner than those of the Holoptychius, which, however, they greatly resemble in their general style of sculpture. Each, on the lower part of its exposed field, was, we see, fretted by longitudinal anastomosing ridges, which, in the upper part, break into detached angular tubercles, placed with the apex downwards, and hollowed, leaf-like, in the centre; while that covered portion which was overlaid by the scales immediately above we find thickly pitted by microscopic hollows, that give to this part of the field, viewed under a tolerably high magnifying power, a honeycombed appearance. The central and lower parts of the interior surface of the scale (a) are in most of the specimens irregularly roughened; while a broad, smooth band, which runs along the top and sides, and seems to have furnished the line of attachment to the creature’s body, is comparatively smooth. The exterior carvings, though they demand the assistance of the lens to see them aright, are of singular elegance and beauty; as perhaps the accompanying wood-cut, (fig. 26,) which gives a magnified view of a portion of the scale immediately above (b) from the middle of the honeycombed field on the right side, to where the anastomosing ridges bend gracefully in their descent, may in some degree serve to show. I have seen a richly inlaid coat of mail, which was once worn by the puissant Charles the Fifth; but its elaborate carvings, though they belonged to the age of Benvenuto Cellini, were rude and unfinished, compared with those which fretted the armor of the Asterolepis.

CRANIAL BUCKLER OF ASTEROLEPIS.

(One fifth nat. size, linear.)]

The creature’s cranial buckler, which was of great size and strength, might well be mistaken for the carapace of some Chelonian fish of no inconsiderable bulk. The cranial bucklers of the larger Dipterians were ample enough to have covered the corresponding part in the skulls of our middle-sized market-fish, such as the haddock and whiting; the buckler of a Coccosteus of the extreme size would have covered, if a little altered in shape, the upper surface of the skull of a cod, but the cranial buckler of Asterolepis, from which the accompanying wood-cut was taken, (fig. 27,) would have considerably more than covered the corresponding part in the skull of a large horse; and I have at least one specimen in my collection which would have fully covered the front skull of an elephant. In the smaller specimens, the buckler somewhat resembles a laborer’s shovel divested of its handle, and sorely rust-eaten along its lower or cutting edge. It consisted of plates, connected at the edges by flat squamous sutures, or, as a joiner might perhaps say, glued together in bevelled joints. And in consequence of this arrangement, the same plates which seem broad on the exterior surface appear comparatively narrow on the interior one, and vice versa; the occipital plate, (a,) which, running from the nape along the centre of the buckler, occupies so considerable a space on its outer surface, exhibits inside a superficies reduced at least one half. Like nine tenths of its contemporaries, the Asterolepis exhibits the little central plate between the eyes; but the eye orbits, unlike those of the Coccosteus, and of all the Dipterian genera, which were half-scooped out of the cranial buckler, half-encircled by detached plates, were placed completely within the field of the buckler,—a circumstance in which they resemble the eye orbits of the Pterichthys, and, among existing fish, those of the sea-wolf. The characteristic is also a distinctive one in Cuvier’s second family of the Acanthopterygii,—the “fishes with hard cheeks.” A deep line immediately over the eyes, which, however, indicated no suture, but seems to have been merely ornamental, forms a sort of rudely tatooed eyebrow; the marginal lines parallel to the lateral edges of the buckler were also mere tatooings; but all the others indicated joints which, though more or less anchylosed, had a real existence. So flat was the surface, that the edge of a ruler rests upon it, in my several specimens, both lengthwise and across; but it was traversed by two flat ridges, which, stretching from the corners of the latero-posterior, i. e. parietal, plates, (b, b,) converged at the little plate between the eyes, while along the centre of the depressed angle which they formed, a third ridge, equally flat with the others, ran towards the same point of convergence from the nape. The three ridges, when strongly relieved by a slant light, resemble not inadequately an impression, on a large scale, of the Queen’s broad arrow.

INNER SURFACE OF CRANIAL BUCKLER OF ASTEROLEPIS.

(One fifth nat. size, linear.)]

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.

PLATE OF CRANIAL BUCKLER OF ASTEROLEPIS.]

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.

PORTION OF UNDER JAW OF ASTEROLEPIS, (OUTER SIDE.)

(One half nat. size.)]

PORTION OF UNDER JAW OF ASTEROLEPIS, (INNER SIDE.)

(One half nat. size.)]

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.

PORTION OF TRANSVERSE SECTION OF REPTILE TOOTH OF ASTEROLEPIS

a. Nat. size.

b. Mag. twelve diameters.]

A. Section of Jaw of Asterolepis.