CHAPTER VII.

Classification of the naturalist not always correspondent with the order of nature—Incongruous grouping of animals in the invertebrate division—Rudimentary skeleton of the cephalopods—Introduction of the vertebrate type into creation—Ichthyolites of the carboniferous rocks—Their state of keeping—Classification of fossil fishes—Placoids—Ichthyodorulites—Ganoids—Their structure exemplified in the megalichthys and holoptychius—Cranium of megalichthys—Its armature of scales—Microscopic structure of a scale—Skeleton of megalichthys—History of the discovery of the holoptychius—Confounded with megalichthys—External ornament of holoptychius—Its jaws and teeth—Microscopic structure of the teeth—Paucity of terrestrial fauna in coal measures—Insect remains—Relics of reptiles—Concluding summary of the characters of the carboniferous fauna—Results.

Theorganic remains hitherto described belong to that large division of the animal kingdom instituted by Lamarck, to comprehend all those whose internal structure is supported by no vertebral column, and which are hence termed invertebrate. They are for the most part protected by a hard outer covering, or exo-skeleton, which assumes many different modifications. We have seen it in the calcareous cells of the little net-like fenestella, in the geometric cup of the stone-lily, in the double case of the cypris, and in the shells of the mollusca. But the order of nature does not always exactly correspond with the classification of the naturalist. His system must necessarily be precise, formal, and defined. One tribe ends off abruptly, and is immediately succeeded by another, with different functions and structure, and dignified with a separate name. But in the order of creation, such abrupt demarcations are few, for if they exist in the present economy, they can not unfrequently be filled up from the existences of the past. There is usually a shading off of one class into another, like the blending of the tints ofsunset, and it often baffles all the skill of the profoundest anatomist, by drawing a distinct line, to pronounce where the one division actually ends and the other begins. Any name, therefore, which is intended to embrace a large section of the animal kingdom, must ever be more or less arbitrary. It will extend too far in one direction, and embrace organisms which might be classed in a different section. It will probably not extend far enough in another, and thus leave beyond its pale animals possessing strong affinities to the majority of those included under it. More especially is this true of every system of classification that proceeds upon the modifications of a single feature, or upon mere negative resemblances. Suppose, for instance, that it were proposed by some highly systematic individual to divide the inhabitants of our country into two great classes—the bearded and the beardless. In the latter category he would arrange all the more quiet and orderly portion of the community, with perchance a tolerable intermixture of rogues. The bearded group would present a most motley array—from the fierce-visaged heroes of the Crimea to the peaceable stone-mason or begrimed pitman—all brought into one list, and yet agreeing in no single feature save that of being like Bully Bottom the weaver, "marvellous hairy about the face." But Lamarck's invertebrate division of the animal kingdom presents a grouping of yet more diverse characteristics, as cannot fail to be confessed when we recollect that it embraces among its members the microscopic monad, the coral polyp, the lobster, the butterfly, the limpet, the nautilus, and the cuttle-fish. Cuvier's three-fold grouping of the division intomollusca,articulata, andradiata, has now supplanted the old name, though the latter is still retained as a sort of convenient designation for all the animals below the vertebrate type.

The most highly developed of the recent cephalopods exhibit a true internal skeleton, in the form of a strong oblong bone, on which the body is hung. In this respect they occupy a sortof intermediate place between the lower molluscs on the one hand, and the lower fishes on the other. Theirs is not a vertebral column, but rather, as it were, a foreshadowing of it; not, however, as a link in some process of self-development from mollusc to fish, for these higher cephalopods do not appear to have been created until fishes and reptiles had lived for ages. The vertebrate type has been traced well-nigh as far back into the past as we have yet been able to penetrate. Once introduced, it has never ceased to exist, but in the successive geological ages has been ever receiving newer and higher modifications, reaching its perfection at length in man. The vertebrate form of structure fulfils the highest adaptations of which terrestrial beings seem capable. We can hardly conceive of corporeal existence reaching a more elevated stage of development, save in thereby becoming less material, and receiving an impartation of some higher element. The vertebrate animals display not merely the most complexly organized structures, but manifest in their habits the workings of the higher instincts and affections. Among the invertebrate tribes the propagation of the species is, in the vast majority of cases, a mere mechanical function, like that of feeding or respiration, and the eggs once deposited, the parent has no further care of her young. But among the vertebrated animals, on the other hand, the perpetuation of the race forms the central pillar round which the natural affections are entwined. It parcels out every species into pairs, in each of which the mates are bound together by the strongest ties of attachment. It gives birth, too, to that noble instinct which leads the mother to expose her own life rather than suffer harm to come to her offspring. It produces, at least in man, that reciprocal attachment of offspring to parent, from which springs no small part of all that is holiest and best in this world. These attributes, to a greater or less extent, belong to all the vertebrate animals, from the fish up to man. In looking over the relics of animal life in the earlier geological formations, we are apt, as we gazeon the massive jaws and teeth, the strong bony armour, and the sharp, barbed spines, to think only of a time of war and carnage, when the larger forms preyed upon the smaller, or ruthlessly sought to exterminate each other. Yet should we not remember, that with all these weapons and instincts of self-preservation there were linked attributes of a nobler kind; that the earliest vertebrate remains point to the introduction—though perhaps in but a rudimentary form—of self-sacrificing love into our planet? The march of creation from the first dawn of life has ever been an onward one, as regards the development not only of organic structure but of the social relations; and if it be true that physical organization finds its archetype in man, it is assuredly no less so that in him too we meet with the highest manifestation of those instincts which, by linking individual to individual, have ever marked out the vertebrate tribes of animals from the more machine-like characteristics of the invertebrate.

We pass now to the vertebrate animals, and shall look for a little into the general grade and organization of the fishes that characterized the carboniferous rivers and seas.

A collection of the ichthyolites of the carboniferous rocks presents almost every variety in the mode of preservation. The smaller species are frequently found entire, and show their shining scales still regularly imbricated as when the creatures were alive. The larger forms seldom occur in other than a very fragmentary condition. The limestones yield dark-brown or black, oblong, leech-like teeth, which are found on examination to be those of an ancient family of sharks. The shales are often sprinkled over with glittering scales and enamelled bones. Some of the coals and ironstones yield in abundance long sculptured spines, huge jaws bristling with sharp conical teeth, and detached tusks, sometimes five or six inches long. In short, the naturalist who would decipher the ichthyology of the Coal formation, finds before him, in the rocks,not a suite of correctly arranged, and carefully preserved skeletons, but a set of disjointed, unconnected bones; here a tooth, there a scale, now a jaw, now a dermal plate, all mingled at random. And yet, though the evidence lie in this fragmentary state, our knowledge of these ancient fishes is far from being correspondingly meagre. To such precision has the science of comparative anatomy arrived, that a mere scale or tooth is often enough to indicate the nature and functions of the individual to which it belonged, and to establish the existence in former times of a particular class or order of animals. Thus the smooth rounded teeth of the mountain limestone are found to present both externally and internally a close resemblance to the hinder flat teeth of the sole living cestraciont (C. Philippi); and we hence learn that a family of sharks, now all but extinct, abounded in the palæozoic seas. The occurrence of a set of dark, rounded little objects, which by the unpractised eye would be apt to be mistaken for pebbles, is in this way sufficient at once to augment our knowledge of the various animals of the Carboniferous period, and to establish an important fact in the history of creation.

Of the four great Orders into which Agassiz[41]subdivided the classPisces, the Placoids and Ganoids, agreeing on the whole with the cartilaginous fishes of Cuvier, occur abundantly in the palæozoic rocks, while the Cycloids and Ctenoids, answering toCuvier's osseous fishes, began in the Secondary formations, and are found in all subsequent deposits. The two former reached their maximum in the earlier geological ages, and have been gradually dwindling down ever since, till now they are represented by comparatively few genera; the two latter are emphatically modern orders; they have been constantly increasing in numbers since their creation, and swarm in every sea at the present day. The carboniferous ichthyolites belong, of course, only to the two first-mentioned orders the placoids and ganoids.

[41]The classification of Agassiz, which is certainly not a little arbitrary and artificial, has been altered by Müller, a distinguished German anatomist, whose arrangement has been modified again by Professor Owen. See Owen'sLectures on Comparative Anatomy, vol. ii. p. 47. There is far from anything like unanimity on the subject. Every naturalist thinks himself at liberty to modify and restrict the groupings of his predecessors or contemporaries, sometimes without condescending to give synonyms or any clue by which one may compare the rival classifications. The geological student cannot engage in a more sickening task than that of ranging through these various arrangements, and he must possess some self-command who can refrain from throwing up the search in disgust. The best way of progressing is to select some standard work and keep to it, until the characteristics of the genera and families have been mastered, and as far as possible, verified from actual observation. After such preliminary training, the student will be more able to grope his way through the "chaos and dark night" of synonyms and systems.

The Placoid, orPlagiostomefishes, are familiar to us all as exemplified in the common thornback and skate of our markets. They are covered with a tough skin, which either supports a set of tuberculed plates as in the thornback, or a thick crop of small rounded bony points or plates, as in the shagreen of the sharks. The head consists of a single cartilaginous box. The spinal column is likewise formed of cartilage, built up in the higher genera of partially ossified vertebræ. The tail is heterocercal or unequally lobed, inasmuch as the spinal column, instead of ending off abruptly as it does in the herring, trout, and all our commoner fishes, passes on to the extreme point of the upper half of the tail. This is a noticeable feature, for it has been found to characterize all the fishes that lived in the earlier geological periods. The fins are often strengthened by strong spines of bone, which stand up in front of them and serve the double purpose of organs of progression and weapons of defence. The teeth vary a good deal in form. In the larger number of existing placoids they are of a sharp cutting shape, often with saw-like edges. Among the sharks they run along the jaws in numerous rows, of which, however, only the outer one is used, those behind lying in reserve to fill up the successive gaps in the front rank. The teeth do not sink into the jaw, as in the ganoids, but are merely bound together by the tough integument which forms the lips. Another form of tooth, abundant among the ancient placoids, and visible on some of thoseat the present day, shows a smooth rounded surface, the teeth being closely grouped together into a sort of tessellated pavement which, in the recent species, runs round the inner part of the jaws, while a row of conical teeth guards the entrance of the mouth.

Fig. 28.—Ctenacanthus hybodoides. (Edgerton.)

The animals which possess these characteristics include the various tribes of the sharks and rays, and form the highest group of fishes. They are all active and predaceous, frequenting every part of the ocean where their prey is to be found. The formidable spines and hideous "chasm of teeth" belonging to the bulkier forms, render them more than a match for any other denizens of the deep, and thus they reign in undisputed supremacy—the scourge of their congeners, and a terror to man.

The seas of the Carboniferous era abounded with similar predaceous fishes, some of which must have been of enormous size. An entire specimen has never been obtained; nor, from the destructible nature of the animal framework, can we expect to meet with one. But the hard bony parts of the animals, those capable in short of preservation in mineral accumulations, are of common occurrence in the mountain limestone beds and even among the coal seams. The dorsal spines orichthyodorulites, are especially conspicuous (Fig. 28). They stood up along the creature's back like masts, the fin which was attached to the hinder margin of each, representingthe sail. The spine could be raised or depressed at pleasure, its movements regulating those of the fin, much as the raising or lowering of the mast in a boat influences the lug-sail that is attached to it. The general form of these spines was long, tapering, and more or less rounded. But they assumed many varieties of surface ornament. Some species were ribbed longitudinally, and had along their posterior concave side a set of little hooks somewhat like the thorns of a rose. Others seem to have been quite smooth, and of a flattened shape, with a thick-set row of sharp hooks down both of the edges, like the spine on the tail of the sting-ray of the Mediterranean. Such weapons have considerable resemblance to the barbed spear-heads of savage tribes, and it is certain they were intended to act in a similar way, as at once offensive and defensive arms. The toothed spines of the sting-rays are still used in some parts of the world to point the warrior's spear and arrow. Is there not something suggestive in the fact that these stings, after having accomplished their appointed purpose as weapons of war in the great deep, should come to be employed over again in a like capacity on the land; and that an instrument, which was designed by the Creator as a means of protecting its possessor, should be turned by man into an implement for gratifying his cupidity and satiating his revenge? Other ichthyodorulites are elegantly ornamented by long rows of tuberculed lines arranged in a zig-zag fashion, or in straight rows tapering from base to point. In all there was a blunt unornamented base, which sank into the back and served as a point of attachment for the muscles employed in raising or depressing the spine. In some specimens the outer point appears rounded and worn, the characteristic ornament being effaced for some distance—a circumstance which probably indicates that these fishes frequented the more rocky parts of the sea.[42]

[42]See Egerton,Quart. Jour. Geol. Soc., vol. ix. p. 281.

The placoid teeth of the carboniferous rocks show the usual forms of the order. Some of them are sharp and pointed, as those of the hybodonts; others have a smooth, rounded, or plate-like form, as in the cestracionts. The latter often show a dark brilliant surface, and might be readily enough mistaken for well-worn pebbles. In the oblong rounded teeth ofpsammodusthe surface is densely covered with minute points like grains of sand, whence the name of the genus. These teeth, when sliced and viewed under the microscope by transmitted light, exhibit a complex reticulated internal structure.

Agassiz' second great Order of fishes is named Ganoid, from a Greek word signifying brightness, in allusion to the brightly enamelled surface of their dermal covering. They differ from the placoids in having their outer surface cased in a strong armature of bone, which is disposed either in the form of large overlapping plates, as among the strange tortoise-like fishes of the Old Red Sandstone, or as thick scales, which are either placed at intervals, as along the back and sides of the sturgeon, or closely imbricated, as in the stony-gar (lepidosteus) of the American rivers. This strong, massive skeleton constitutes in many genera the sole support of the animal framework, the inner skeleton being of a gristly cartilaginous kind, like that of the skate. On this account traces of the vertebral column are by no means abundant among the older formations. But as the ganoids form a sort of intermediate link between the placoid or gristly fishes on the one hand, and the bony fishes on the other, they are found to present in their different genera examples of both these kinds of structure. Thus, the skeleton of the sturgeon consists of a firm cartilage, out of which the vertebræ are moulded, so that this fish was at one time ranked with the sharks in the cartilaginous tribe of Cuvier. The skeletons of some of the older ganoids (asholoptychius), on the other hand, manifest such a decidedly osseous structure, with sometimes so much of a reptilian cast, that the bones were at first referred to some hugeextinct saurians. The head of the ganoid fishes is encased in a set of large massive plates of bone, and the jaws are furnished with several rows of small sharp teeth, intermingled with a less numerous but larger-sized and more formidable kind. The interior of the mouth likewise displayed in many ancient genera groups of palatal teeth, so that the dental apparatus of these animals must have been very complex and complete. The tail in all the older ganoids was heterocercal, like that of the sharks, the lobes being not unfrequently densely covered with minute overlapping scales of bone—a peculiarity which also extended to the fins. But the fins were sometimes strengthened in another way by having the foremost ray greatly thickened and enlarged, so as to form a stiff spine like the ichthyodorulites of the placoids. The whole of the external surface of these ganoidal fishes glittered with enamel, and was usually sculptured in the most graceful patterns or ornamented with fine lines and punctures so minute as to be almost invisible to the naked eye. Every plate, scale, fin-ray, nay, the very lips exhibited the characteristic enamel mottled over with the style of ornament peculiar to the species. And when we think we have exhausted the contemplation of these beauties, it needs but a glance through an ordinary microscope to assure us that the unassisted eye catches only a superficial glimpse of them. The more highly we magnify any portion of these old-world mummies, the more exquisite does its structure appear.

In the carboniferous rocks of Great Britain, upwards of forty species of ganoids have been detected. They have a wide range in size, the smallest measuring scarce two or three inches, while the largest, to judge at least from the bones which they have left behind, must have reached a length of twenty, or perhaps even thirty feet The lesser genera (Fig. 29) were characterized by small, angular, glossy scales, usually ornamented either with a very minute punctulation, or with fine hair-like lines which sometimes exhibited the most complicated patterns.The scales were likewise occasionally serrated along the exposed edges—a style of ornament which gives no little richness to the aspect of the dermal covering. The fins, closely imbricated with small angular scales of bone, sometimes displayed a striated ray in front, but this neither possessed the strength nor the formidable aspect of the corresponding spine among the placoids. The head was encased in a set of bony plates fitting tightly into each other, and ornamented with various patterns according to the species. The teeth were very small and fine, resembling the bristles of a brush, but in at least some species intermingled with teeth of a larger size. The minute style of dentition in these smaller fishes has been thought to indicate their habit of keeping to the bottom of the water and feeding on the soft decaying substances lying there. Nowhere have I seen the small rhomboidal scales of thepalæoniscusso abundant as among dark shales charged with cypris cases and fragments of terrestrial plants, and on such occasions the idea has often occurred that these graceful little fishes, like theamiaof the American rivers, may have fed on the cyprides that swarmed along the bottom of the estuary.

Fig. 29.—Amblypterus macropterus (a Carboniferous ganoid).

Scattered over the fresh-water limestones, ironstones, and shales, or crowded together along the upper surface of some ofthe coal-seams, there occur the remains of two very remarkable ganoidal fishes. They deserve our attention for their great size, their complex organization, and the important place in the scale of animal life which they occupied during a former period. One of them has been calledmegalichthysorgreat fish—an unhappy name, since the animal did not reach the dimensions attained by not a few of the other ganoids, and was even surpassed by at least one of its contemporary congeners. The other is known as theholoptychiusorwrinkled scale. A more detailed examination of these two animals will perhaps best enable us to understand the character of the ganoid fishes that lived in the waters of the Carboniferous period.

The megalichthys had an average length of about three feet. Like the other members of the ganoid order it had a glittering exterior, every scale and plate being formed of strong bone, and coated with a bright layer of enamel. Wherever this polished surface extends, it is found to be ornamented with a minute punctulation, the pores of which lie thickly together like the finer dots of a stippled engraving. The cranial plates are further varied by a scattered and irregular series of larger punctures that look as if they had been formed by the insertion of a pin-point into a soft yielding surface. The examination of the head of the megalichthys as depicted inFig. 30, will convey an adequate conception of the structure of a ganoidal cranium.

Fig. 30.—Head of Megalichthys Hibberti, one-sixth of natural size (Agass.Poiss. Foss.Tab. 63).A Upper side. B. Under side. C Profile.

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 theosteolepisof 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 wasbut 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'sFootprints 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 fibreswhich 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 ahard 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 hisFirst 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 livelydiscussion 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 undescribedfish. 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'sBridgewater 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. 89et seq.

The wordholoptychiusmeans, 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.

But perhaps the most remarkable and characteristic parts of the carboniferous holoptychius were its jaws and teeth. As we might readily conjecture from the great size and strength of the scales and cranial plates of this fish, its dentition was of a correspondingly massive type. The under jaw, with the usual corrugated ornament, frequently exceeded a foot in length, and displayed along its upper edge a thick-set group of teeth. Of these there were two kinds one of a smaller size and more blunted form, with short indented furrows at their base; the other of a greatly more formidable size, grouped at intervals among the smaller ones. The front end of each under jaw bore one of these long conical tusks, serving as it were to guard the entrance of the mouth. Each of the larger teeth had a base strongly marked with longitudinal furrows, and sank deep into the jaw, with the bone of which it sometimes anchylosed.[47]The part of the tooth above this socket had an oval form, so flattened as to present two cutting edges, one facing the front, the other the back of the mouth, and meeting at the upper end of the tooth which was sharp and pointed. Such large conical tusks may frequently be obtained, having a length of two orthree inches, while occasionally they range as high as six or seven, the smaller teeth seldom reaching so much as an inch. It is difficult to see how, with such a formidable dentition, the jaws could readily close. In some specimens I have seen deep hollows beside the bases of the teeth, which may possibly have received those of the opposite jaw, but the gigantic tusks at the entrance of the mouth seem to have stood high over the jaw, passing outside like those of the wild-boar. If this be correct, the jaw of the holoptychius would unite the mechanism of both the alligator and the crocodile—its recipient hollows being analogous to the tooth-pits in the former tribe, and its protruded teeth to the similarly exposed teeth of the latter.

[47]I have seen detached teeth, wherein the length of the root, or part imbedded in the jaw, tripled that of the exposed part, sinking four or five inches into the bone without any trace of anchylosis. Whether these huge tusks belonged to the upper or under maxillary, I do not pretend to say, though no specimen of the under jaw, which has ever come under my notice, would accommodate half of such a deep-sunk base.

Fig. 31.—Jaw of Holoptychius (Rhizodus.—Owen) from Gilmerton, one-fourth nat. size; the large teeth along the middle part of the jaw are here wanting.

When we bring the microscope to bear upon the elucidation of the structure of these ancient teeth, it seems as if our labour had but just begun; and that so far from having by an external scrutiny exhausted all that they have to show us, our knowledge of them can be but scanty and superficial until we have studied them carefully under a magnifying power. Microscopic sections of such organic remains are prepared in the same way as those of the fossil woods already noticed; and a more interesting or beautiful series of objects cannot be conceived than a set of slices of these fossil-teeth.

Viewed, then, in longitudinal section from base to point, the part above the fluted root of one of the large teeth of the holoptychiusis seen to consist of minute hair-like fibres of extreme tenuity, which proceed in straight lines from the outer surface to the interior. At right angles to these, and parallel with the outer edges, there is a set of dark widely-placed lines conforming to the outline of the tooth, like so many long sugar-loaf shaped caps, placed within each other. When this part is cut across, and viewed in transverse section, the tooth is observed to be of a flattened oval form, with the same fine fibres or tubes radiating from the centre, and traversed by the same dark bands which now assume the form of concentric rings. The appearance thus presented reminds one at once of a cross section of some dicotyledonous tree, the dark bands resembling the annual layers of growth, and like these resulting from a similar thickening of the internal tissue. The upper part of the tooth is solid and the concentric rings few; the middle exhibits an increase of the rings, and possesses, moreover, a hollow centre or pulp-cavity,[48]with the usual diverging fibres. Here the oval form is well shown, and the encircling rings are considerably flattened at the ends of the long axis.

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