Fig 27.—Carboniferous Cephalopods.
1. Nautilus Koninckii. 2. Goniatites crenistria. 3. Orthoceras laterale (fragment).
The Cephalopods, or highest class of mollusca, are represented among the British carboniferous strata by seven genera. Of these the most characteristic is the orthoceras, so named from its shell being like a long straight horn. When the animal was young it inhabited a single-chambered shell like that of many of the gastropods, but as it increased in size and prolonged its shell in a straight line, it withdrew from the first occupied chamber. This was partitioned off by a thin wall called a septum, through the centre of which a tube ran to the narrow end of the shell ([Fig. 27]). As the creature grew, chamber after chamber was in this way formed, each of them quite air-tight, and traversed by the central tube. Suppose a graduated series of diminutive watch-glasses to be pierced by a long tapering glass-tube in such a way that they should have their convex faces towards the narrow end of the tube, and be arranged at short intervals, the smallest one placed near the point of the tube, and the largest a little below the wider end. Suppose, further, that this piece of mechanism were placed within another tube tapering to an obtuse point, and that the edges of the watch-glasses fitted tightly to the inner surface of this larger tube. Such would be a rough model of the structure of the orthoceras.
The inner tube that traverses the centre of the chambers from end to end of the shell is called the syphon, but its uses are very problematical. At one time naturalists inclined to regard it as intended to be filled with fluid, which, by expanding the membrane of the tube, would compress the air in the chambers, and thus, increasing the specific gravity of the animal, enable it to sink to the bottom. In this way, by emptying or filling the syphonal tube, the orthoceras might have risen rapidly to the surface of the deep, or sunk as swiftly to the bottom. But this view, so pretty that one wishes it were confirmed, must be regarded as at least doubtful. The orthoceras more probably owed its power of progression to the action of a funnel connected with the breathing apparatus, whereby jets of water were squirted out that drove the shell rapidly along. The use of the air-tight chambers was, perhaps, to give buoyancy to the shell so as to make it nearly of the same specific gravity as water. Such a provision must have been amply needed, for Professor Owen mentions an orthoceras from Dumfries-shire that measured six feet in length, and similar gigantic specimens have been found in America. Unless the chambers in these shells had been air-tight, the animals that inhabited them would have been held down about as firmly to one spot as if they had been tied to a sheet-anchor. No mollusc could have possessed much locomotion with so ponderous a tail, six feet or more in length, to drag after it. But this inconvenience was obviated by the simple plan of having the chambers close, and filled with nitrogen or other gas evolved by the chemistry of the inmate. The shell, in this way, acquired no little buoyancy, and probably stood up like a church spire, the animal keeping close to the bottom to lie in wait for any hapless mollusc or trilobite that might chance to come in its way.
The nautilus ([Fig. 27]), which still lives in our seas, occurred likewise in those of the Carboniferous period. It was a coiled shell; in truth, just an orthoceras rolled up in one plane like a coil of watch-spring. An allied form, called the goniatite ([Fig. 27]), had the margins of its septa of a zig-zag form, like the angles of the wall round a fortified town. When the thin outer coating of the shell is removed, the ends of these partition-walls are seen to form strongly-marked angulated sutures or joints, where they come in contact with the shell. Hence the name of the genus—angled shell.
All these animals were predaceous. They did not confine themselves to the lower forms of life, polyps and medusæ, nor even to the humbler tribes of their own sub-kingdom, but hesitated not to wage war with creatures greatly higher in the scale of creation than themselves, such as the smaller fishes. They swarmed in the palæozoic seas, and well merited the title of scavengers of the deep, that has been bestowed on the sharks of our own day. They seem to have performed a function now divided partly among the fishes and partly among the higher gastropodous molluscs. And accordingly we find that as these latter tribes increased, the orthoceratites, and goniatites, and ammonites waned. At the present day, of all the palæozoic cephalopods there remains but one—the nautilus[40]; a and so rare is it, that up to the year 1832, all sorts of fanciful notions existed as to its nature and functions. In fact, the nautilus was a sort of myth which any naturalist could dress up as he chose, much as the old poets used to picture the ship Argo. A specimen was at length procured and intrusted to the examination of Professor Owen, by whom its anatomy was studied, and afterwards philosophically described in an elaborate monograph. Then, for the first time, did geologists obtain a true notion of the nature of those siphonated shells, which lie grouped by hundreds in the palæozoic and secondary formations. Yet we still want an account of the habits of the nautilus. The older naturalists alleged that it could at pleasure rise to the surface or sink into the depths of the ocean; that it could spread out its fleshy arms and float across the waves or draw them in, capsize the little vessel, and so return to a creeping posture among the sea-weed at the bottom. These statements may to some extent be true, for the chambers of the nautilus shell must impart great buoyancy to it. But in the meantime the story of the sailing propensities of the animal is derived from a sort of mythic age, and must be viewed with some little suspicion. Until further observations are made, we shall neither fully understand the economy of the nautilus nor the habits of the cephalopods of the palæozoic seas. But the day is probably not far distant when such doubts will be set at rest, and we shall know whether the nautili and orthoceratites swam in argosies over the surface of the ocean, or, keeping ever at the bottom, left the waves to roll far above them, unvaried save perchance by some floating sea-weed or drifted tree.
[40] And perhaps even that is doubtful, for it is not unlikely that after all, the palæozoic nautili may belong in reality to another genus. Twenty years hence will probably see no little change on our present identifications.
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.
The organic 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 of sunset, 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 into mollusca, articulata, and radiata, 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.