The snout is formed of an elegantly curved bone (c) fringed along its under edge with minute thick-set teeth. On either side it is flanked by two triangular plates, which occupy the space between the intermaxillary bone (c) and the upper jaws (q q). The eye orbits seem to have been at the corners of the intermaxillary, circumscribed by the sub-orbitals (f g h) and the ethmoids (b). The massive intermaxillary bone had its posterior margin of an angular form, and into the notch thus formed there was wedged the anterior end of a long strip of plates, which expanded as they approached the occipital part of the cranium, and terminated in three irregular plates that may represent the place of the parietal and occipital bones. The space between this belt and the upper jaws was occupied by three large plates (i k l) which in other ganoids, as the osteolepis of the Old Red Sandstone, were united into a single pre-opercular bone of considerable size. The operculum or gill cover (m) was relatively large, and had an elegantly curved anterior margin. The upper jaws (q) were comparatively small, and had a fringe of small conical teeth. The under jaws (r r) reached to nearly double the length of the upper, and were similarly set round with teeth. The teeth of the megalichthys, like those of the living lepidosteus, consisted of two kinds, of which the one bristled thickly along the outer edge of the jaw as sharp minute points, averaging about a line in length, while behind this outer row lay a scattered series of much larger teeth that sometimes rose nearly an inch above the jaw. The external surface of these more formidable tusks is smooth, glittering, and minutely striated with fine lines from base to point, while the root of each is farther marked by a circle of short, deep, longitudinal furrows. The internal structure displays a close ivory, which when viewed under a microscope is seen to be made up of fine tubes radiating from the outer surface to the hollow central cavity. Some of the bones in the interior of the mouth seem to have been also furnished with an apparatus of teeth. The under surface of the cranium between the arch of the under jaws consists of two oblong central plates (t) surrounded by a row of sixteen irregular ones, eight on each side, and terminated in front by a large lozenge-shaped scale (u) which fits into their angle of junction on the one side, and into the symphysis of the jaws on the other. In the osteolepis there were likewise two large plates terminating in a similar lozenge-shaped one, but without the flanking rows. In the famous Old Red holoptychius of Clashbennie, the under surface of the head had but two plates, and in the still older and more gigantic asterolepis, there was but one. It is the delightful task of the paleontologist to compare and contrast these various pieces of mechanism, to mark how what seems lacking in one comes to be supplied in another, and to trace out the various modes in which, during the ages of the past, Nature has wrought out the same leading plan, sounding, as it were, an ever-changing series of modulations upon one key-note. In comparing together the ganoids of the Old Red Sandstone and the Carboniferous rocks, he finds that in the asterolepis—a fish belonging to the lower part of the former formation—the pointed arch formed by the sweep of the lower jaws is filled up by a single plate like some abbey-window with its mullions knocked away, and built up with rude stone and lime. Higher in the same group of rocks he meets with the cranium of the holoptychius, where there is one straight central mullion running in an unbroken line from the angle of the arch to its base. In the osteolepis[43] he sees this mullion branching into two at its upper end, so that the window consists of three divisions, as in the simplest style of Gothic. Passing upwards into the Carboniferous system, he encounters a still more ornate arrangement in the cranium of the megalichthys. The central mullion with its two upper branches still remains, but it is flanked by an additional one on each side, from which there spring six cross bars that diverge obliquely with a slight curve, so as to join the outer arch and subdivide the window into nineteen compartments. So varied are the plans of the Divine Architect in what to man may seem such a little matter as the piecing together of a fish's skull.
[43] Hugh Miller's Footprints of the Creator, p. 91.
The body of the megalichthys was cased in an armature of as solid and glittering bone as that which defended its head. Where the plates of the cranium ended off they were succeeded by large rhomboidal scales that crossed the body obliquely, and overlapped each other like the metal plates in the antique scale-armour. Each scale consisted of two parts, of which one had a rhomboid form and was covered over with enamel, while the other ran round the two inner sides of the rhomb as a broad unenamelled selvage deeply indented along its centre. It was the enamelled portion alone that formed the outer surface, the rough unpolished border being covered by the overlapping edges of the adjoining scales. The scales had not a uniform thickness, but were strongest at the covered part from which each thinned off to the outer edges. In this way the thin edge of one scale pressed down on the thick part of the subjacent one, and a covering of uniform strength and smoothness was produced. Looking at a set of these scales as they still occupy their original position on the creature's body, it is scarcely more than a half of each which meets our eye; for the unenamelled border occupied about a third of the entire surface, and a fourth of the remainder was covered by the overlapping scales. The effect of this arrangement must have been to combine great strength with the most perfect flexibility. Notwithstanding the bulk of his helmet and the weight of his scale-armour, we cannot conceive the megalichthys to have been other than a lithe, active, predaceous fish, dealing death and destruction among the herring-like shoals of little palæonisci and amblypteri, though able to maintain perhaps but a doubtful warfare with his more bulky contemporary, the holoptychius. The internal structure of the scales of the megalichthys exhibits the same provision for combining strength with the least possible amount of material. Viewed in a transverse section under a magnifying power of about eight diameters, they are seen to consist of three layers of bone; each possessing a peculiar structure. The outermost is formed of a tessellated pavement of minute round ocelli, having a fine brown colour, and placed close together with considerable regularity. They somewhat resemble little wheels, the axle being either a dark solid nucleus or a small circular aperture, whence there radiates to the outer rim a set of exceedingly minute fibres which were originally hollow, and served as canals to carry on the growth of the scale. The vacant space left where four wheels impinge on each other, forms one of the pores that cover the enamelled surface of the scale. The whole structure of this outer layer very closely resembles that presented by the internal part of the base of the teeth, save that the confluent lobes shown in the teeth become in the scale detached into separate and independent circles. The central stratum of each scale is composed of a loose open network of cancellated bone that passes into the layer on either side, and resembles in its general texture the osseous vertebræ of the same fish. The under layer, one end of which rested immediately on the skin, approaches more to the firmness and solidity of the outer one, but, in place of a tessellated, ivory-like pavement, it had a close fibrous texture, with here and there a scattered cavity, and the fibres were matted together so as to resemble the more solid structure of the cranial bones. The effect of this triple arrangement must have been to impart great strength and lightness to the external armature of the fish; the middle spongy layer serving, by its porosity, at once to deaden the effect of any blow aimed at the outside, and to give buoyancy and lightness to what would otherwise have been a coat of mail well-nigh, as ponderous as that of a feudal chief. One can hardly conceive any implement of warfare in use among the lower animals of strength enough to pierce this massive covering. But we shall find as we go on that if the megalichthys had a strong defensive armour, a bulkier neighbour had a still stronger offensive one, and that the enamelled plates of the one fish were scarcely a match for the huge pointed tusks of the other.
The megalichthys had an osseous skeleton, with vertebræ of a discoidal form. These internal bones when viewed under the microscope are found to display an open cancellated structure, resembling that of the central layer in the scales. It thus appears that this ancient fish was not merely defended by a hard external armour, but possessed an equally solid framework of bone within.
Mingled with the scales and bones of the megalichthys, there are found the remains of a still larger fish, to which the name of Holoptychius has been given. Its external ornament differed entirely from that of the animal last described. It possessed teeth sometimes six or seven times larger, and jaws, plates, and bones of a form and dimensions totally distinct. Strange as it may seem, however, these two fishes have been constantly and systematically confounded from the time when they were first discovered. Two or three years ago, there might be seen in the British Museum several specimens of the holoptychius, of which some bore the correct name, while the rest were labelled "Megalichthys;" and a similar error prevailed in several of the other museums.[44] The confusion can be traced very distinctly in the memoir of Dr. Hibbert, who for the first time described the remains of these fishes, and wrote according to information received from Agassiz.
[44] The mistake was noticed in 1845 by Hugh Miller, who, in a foot-note to his First Impressions of England and its People, p. 71, well defines the distinctions between the two ichthyolites.
In the autumn of 1832, the attention of the scientific public of Edinburgh was directed to the extraordinary character of some fossil remains obtained from the lime-quarries of Burdiehouse, a village about four miles to the south of the town. Dr. Hibbert visited the locality, and soon saw enough to excite his lively interest in its thorough investigation. The Royal Society of Edinburgh warmly supported his exertions, and by their means a large suite of specimens was eventually obtained, which the Doctor from time to time described as they were successively received. At the meeting[45] of the British Association in Edinburgh, in 1834, the specimens were exhibited before the Geological Section, and a memoir upon them read by their successful discoverer. On the conclusion of the paper, a lively discussion ensued upon the nature of the animal to which the scales and teeth had belonged. Dr. Hibbert argued, from the deeply-furrowed teeth, and the strong, massive cranial plates, that the animal must have been a reptile, and supported his assertion by no small amount of anatomical skill. In the midst of the discussion, a message was sent to the great ichthyologist of Neufchatel, who happened to be at that time busily engaged in the Zoological Section. Passing over the fossils as they lay grouped upon the table, with that quick perception for which he is so justly celebrated, Agassiz at once decided that the bones must have been those of some large and hitherto undescribed fish. Such a decision from such an authority produced of course no little sensation, and the naturalist was told with some surprise that the remains had just been elaborately described as those of extinct reptiles. "Reptiles!" thought Agassiz, and again his quick eye darted over the table; but the fossils would yield no other answer than what they had already given. Despite their seeming reptilian character, they were undoubtedly ichthyic, though belonging to an animal up to that time unknown. In the completed memoir which Dr. Hibbert subsequently submitted to the Royal Society, his mistake was freely acknowledged, and the remains there flourish as those of a true fish. But with this amendment a grave error of another kind was committed, though in this the Doctor seems to have been supported by the authority of Agassiz himself. The large bones, scales, and teeth of the Burdiehouse limestone, were all indiscriminately thrown into one genus, to which Agassiz gave the name of Megalichthys; and in the memoir we find the different kinds of scales and teeth described and figured without the slightest intimation or suspicion that they might possibly have belonged to different animals. The novelty of the discoveries soon attracted general attention to Dr. Hibbert's paper. It was quoted or referred to in almost every scientific work treating of general geology, while in some instances (as in Dr. Buckland's Bridgewater Treatise) the erroneously-named bones were re-engraved. A tooth from the Fife coal-field, drawn for one of the woodcuts in a popular elementary manual, was also named megalichthys; an error perpetuated through every edition till the last, where the tooth has been restored to its true owner—the holoptychius. In truth, no two organisms have ever been so maltreated; and if the reader will kindly bear with me a little further, it will not be difficult to show him that the holoptychius had peculiarities of its own quite as distinct as those that have come before us in the megalichthys, and that each animal has a full and legitimate claim to a separate and independent niche in the gallery of fossil fishes.
[45] See Agassiz, Poiss. Foss., tom. ii. Part 2, p. 89 et seq.
The word holoptychius means, as I have said, "wrinkled or folded all over,"—a name truly expressive of the peculiar style of ornament displayed by every part of the exterior of the animal's body. The head-plates, which are of great size, exhibit a fine corrugated shagreen-like surface, roughened into knobs, and wavy lines of confluent tubercules, that remind one disposed to be fanciful, of a frosty December moon with its isolated peaks, and confluent mountain chains. The scales are of a rounded or oval form, and vary from less than half an inch to fully four or even five inches in diameter. Their upper side consists of two parts, one of which with a crescent shape lay beneath, the over-lapping scales, while the other passed outwards to form a portion of the outer visible surface. The part that was hidden by the overlapping scale was smooth, with a finely striated surface. The exposed portion displayed the usual corrugated sculpturing, many of the little tubercules having striated sides, and showing, in consequence, no little resemblance to the star-like knobs on the dermal covering of the Old Red Sandstone asterolepis. The inner surface of the scales was concave, with a central prominent oblong point surrounded by encircling scaly ridges, and forming what is called the centre of ossification.[46]
[46] The above descriptions of the scales and teeth of these two fishes, are taken from specimens in my own collection. None of my holoptychian scales show incontestable the proportion of the covered to the exposed part. Judging from the aspect of one of them, the wrinkled portion occupied perhaps about three-fifths of the entire scale, the remaining part being covered by the overlapping edges of those adjacent; for the characteristic corrugated surface was essentially an external ornament, and ceased at the point where the external bone passed into the interior. I may remark, that the upper side of the scales is not very frequently seen in the Burdiehouse limestone, the rough surface usually adhering to the rock, and leaving only the smooth inner side exposed. Out of seven specimens from that locality, only one shows the upper side, and that by no means in a perfect state of keeping. The structure alike of scales and bones can be seen to much greater advantage in the shales, ironstones, and coals of the coal-fields, where, owing to the soft nature of their matrix, the fossils can be readily cleared and exposed.