CHAPTER VI.

Carboniferous fauna continued—George Herbert's ode on "Man"—His idea of creation—What nature teaches on this subject—Molluscous animals—Range of species in time proportionate to their distribution in space—Two principles of renovation and decay exhibited alike in the physical world and the world of life—Their effects—The mollusca—Abundantly represented in the carboniferous rocks—Pteropods—Brachiopods—Productus—Its alliance with Spirifer—Spirifer—Terebratula—Lamellibranchs—Gastropods—Land-snail of Nova Scotia—Cephalopods—Structure of orthoceras—Habits of living nautilus.

HolyGeorge Herbert, in one of the most remarkable odes of the seventeenth century, sang quaintly, yet nobly, of the dignity of man. He looked into the design and nature of the human heart, and saw there a palace that had been built for the abode of the Eternal. Deserted though it might be, broken down and in ruins, yet there still lingered a trace of its ancient glory, and the whole material world still testified to its inherent greatness. He looked abroad on the face of nature, and saw, in all its objects and all its movements, a continued ministration to man.

"For us the windes do blow;The earth doth rest, heav'n move, and fountains flow.Nothing we see, but means our good,As our delight, or as our treasure;The whole is, either our cupboard of food,Or cabinet of pleasure."The starres have us to bed;Night draws the curtain, which the sunne withdraws;Musick and light attend our head.All things unto our flesh are kindeIn their descent and being; to our mindeIn their ascent and cause."

"For us the windes do blow;

The earth doth rest, heav'n move, and fountains flow.

Nothing we see, but means our good,

As our delight, or as our treasure;

The whole is, either our cupboard of food,

Or cabinet of pleasure.

"The starres have us to bed;Night draws the curtain, which the sunne withdraws;Musick and light attend our head.All things unto our flesh are kindeIn their descent and being; to our mindeIn their ascent and cause."

"The starres have us to bed;

Night draws the curtain, which the sunne withdraws;

Musick and light attend our head.

All things unto our flesh are kinde

In their descent and being; to our minde

In their ascent and cause."

The idea is a very natural one, and is consequently as old as man himself. Human vanity is soothed by the reflection that all this varied world, with its countless beauties, has been designed and arrayed solely for the use of man. And yet, if we but think of it, such a view of creation, however natural and pleasing, is at the best but a narrow and selfish one. It assuredly finds no response in nature, and grows more and more out of fashion the further our investigations proceed. Nature teaches us that long ere man appeared upon the earth there were successive generations of living things just as now; that the sun shone, and the waves rolled, and the wind blew, as they do to-day; and that, on as lovely a planet as that whereon we dwell, there lay forests and prairies nursing in abundance animals of long-extinct forms; lakes and rivers, haunted by creatures that find no representatives now; and seas teeming with life, from the minute infusory up to the most unwieldy icthyosaur, or the most gigantic cetacean. And all this, too, ere a reasoning, intelligent being had been numbered among terrestrial creatures, and when, perhaps, each successive creation was witnessed by none save those "morning stars who sang together, and those sons of God who shouted for joy." The delight and comfort of the human race formed, doubtless, one of the many reasons why this globe was so bountifully garnished.[32]But the workmanship of a Being infinitely wise, and good, and powerful, could hardly have been other than complex and beautiful. That symmetry and grace which we see running as a silver thread through every part of creation, forms one of the characteristicsof the Almighty's mode of working. From the Fountain of all Beauty nothing unseemly or deformed can proceed. And so we find, away back among the ages of the past, that, though the material world might be less complete, it was not less beautiful than now. Nay, those bygone millenniums stood higher in one respect, for the eye of God rested upon their unsullied glory, and he pronounced them very good; but these last ages of creation are dimmed and darkened, and that Eye now watches a world trodden down by the powers of evil. There is profound truth in the sublime allegory of Milton that represents Sin girt round with clamorous hell-hounds, and the two grisly forms sitting at the farthest verge of purity and light, to keep the gates of darkness and chaos. With the introduction of moral evil into our planet came the elements of deformity and confusion. The geologist can go back to a time ere yet the harmony of nature had been broken. The Christian looks forward to a day when that harmony shall be again restored, and when guilt with all its hideous train shall be for ever chased away from the abodes of the redeemed.

[32]In connexion with this subject I have been often struck with a passage in St. Paul's Epistle to the Colossians, i. 10, "All things were created by him [Christ] and for [εις—with a view to, on account of] him." It is probable that these words, in their full meaning, cannot be understood by us. Yet they seem to point to Christ as at once the Creator, and himself the acme and design of creation; and perhaps they may contain what hereafter shall prove the key to the mystery of creation. On this impressive and difficult subject the reader should refer to the closing chapter of Hugh Miller'sFootprints of the Creator. See also M'Cosh onTypical Forms, 2d edit. p. 531.

Such thoughts as these sometimes arise in the mind of one who labours much among organic remains. By no class of fossils are they more vividly suggested than by those which we come next to examine—the various tribes of molluscous animals. This results from the high antiquity of these organisms, and the similarity of type which they have manifested in all ages. In the very earliest geological periods they exhibited the same symmetry of external form as now, the same beauty of structure, and apparently the same delicacy of colour. Nay, so closely did they resemble their existing congeners that we are seldom at a loss as to their affinities, and can refer them to their places in the scale of creation, and sometimes even to genera still living.[33]

[33]It must be admitted, however, that not a few of the identifications already made are somewhat suspicious The natural tendency is to perceive resemblances—a tendency which even the most rigid science sometimes fails to control.

The geological ages saw many strange types of creation. One era, in especial, furnished reptiles which united in their structure the snout of the porpoise, the head of the lizard, the teeth of the crocodile, the paddles of the whale, and the backbone of the fish. Some displayed the long pliant neck of the swan, and others careered through the air on wings like those of the bat. But the molluscous tribes have never exhibited such aberrant forms. The existing classes and orders of the naturalist are still the same as those which nourished during the successive geological periods. Hence their value as evidence of physical changes in the ancient world. Hence, too, the conviction, forced upon the mind of the observer, that the conditions for the support of life never deviated much from those now in operation; that in place of all the varied beauty of the world having arisen for the use of man, it existed millions of years ere the breath of life had been breathed into his nostrils; that in fine, man is but a new-comer, a creation of yesterday.

There is another point suggested by the occurrence of mollusca in the Carboniferous system, to which it may be well to refer, namely, the curious, and as yet not wholly understood fact, that the range of animals in time is in some way proportionate to their range in space. In other words, it often happens (so often, indeed, as apparently to indicate a law) that the more widely diffused a genus is found to be at the present day, the farther back can we trace its remains into the geological ages. This fact probably depends upon causes, many of which are still unknown to us; but the following remarks may help the reader to a notion of the general bearings of the subject.[34]

[34]The law is more especially exemplified by the mollusca, but it may eventually be found to characterize other classes. We, perhaps, see traces of it in the present distribution of the two most ancient orders of icthyic life—the placoids and ganoids.

In the profounder recesses of the ocean, the temperatureremains more or less uniform all over the globe.[35]In these undisturbed regions there occur, along with corals and other humble animals, many kinds of mollusca, such as terebratulæ, craniæ, scissurellæ, &c. These are very generally found not to be confined to one province or limited district, but to flourish in every sea from Hudson's Bay to Hindustan. One of the causes of this wide distribution is the uniformity of temperature that characterizes the depths in which they live. They can migrate from one ocean to another, from the torrid zone to the polar circle, without experiencing any destructive change in the thermal conditions of their element. And provided only they meet with no barrier in the form of a lofty submarine mountain chain or profound abyss, and can secure the requisite food in their journey, we know no reason why some of these shells may not thus extend themselves over wide areas. Of the two species ofrhynconellanow living, one inhabits the depths of the icy sea, the other enjoys the warmer waters that lave New Zealand. The species, in this case, seem (for the fact cannot yet be accepted as fully proved) to occupy a more limited area, while the genus has a larger range.

[35]The stratum of constant temperature runs in a wave-like form from pole to pole. In the arctic and antarctic oceans it is found at a depth of 4500 feet, whence it slopes upwards so as to reach the surf ice at the temperate zone on both sides of the equator. It then gradually sinks down in the warmer regions, till at the equator it is 7200 feet below the sea-level. There are thus one tropical and two polar basins separated by two wave-like circles, or, as a geologist would say, three synclinal troughs separated by two anticlinal ridges.

Now, a genus widely diffused, and capable of enduring great differences in the temperature and other conditions of the ocean, would probably suffer least from any great physical changes. If all the sea at one locality were converted into land, the genus would be driven into other districts, and thrive as abundantly as ever; or, even supposing that it should become locally extinct, it would still be abundantly represented in other oceans of the globe. In the course of many ages, aftermany such slow revolutions in the configuration of land and sea, the genus might perhaps become greatly reduced in numbers, until at length some final elevation of the sea-bed, or other change, might cause its total extinction. In therhynconella, we perhaps see one of these genera in its last stage. Any great change in northern latitudes would probably destroy the arctic species, and a similar change around New Zealand might gradually extinguish the southern one.

Looking, then, from this point of view into the past history of life upon our planet, we see that such extinctions have often taken place. At first, many of these widely-diffused genera were created. They were represented by a large number of species as well as individuals, and ranged over all the oceans of the globe; but in tracing out their history, we mark one species after another passing away. Some of them lived for but a comparatively short period; others came in with the beginning and saw out the end of an entire geological system; but of all these early species there is not now a single one extant, though some of the genera still inhabit our seas. It is plain, therefore, that but for the operation of another principle, all the genera, too, would ere this have become extinct, for the whole can contain no more than the sum of its parts; and if these parts are destroyed the whole must perish simultaneously. As the species of certain genera died out, however, their places were from time to time filled up with new ones, yet the rate of increase became ever less and less than the rate of decrease, so that the numbers of such genera grew fewer with every successive period, and have reached their minimum in existing seas. There are instances, however, in which this ratio was reversed, the list of added species continually outnumbering that of the extinct, till the genus reached its maximum, when it either continued at that stage till the present day, or began slowly to decline.

In the physical world around us, we behold a perpetual strife between the two great principles of renovation and decay. Hillsare insensibly crumbling into valleys; valleys are gradually cut down, and their debris transported to the sea. Our shores bear witness to the slow but ever onward march of the ocean, whether as shattered cliff's worn by the incessant lashing of the surge, or as sand-banks and submerged forests that represent the wolds and holms of our forefathers. We mark, too, how the sediment thus borne into the main is sowing

"The dust of continents to be;"

while the slow elevation of large tracts of country, or the sudden upheaval of others, shows us by how powerful an agency the balance of land and sea is preserved, and how sometimes the paroxysm of an hour may effect a mightier change than the wasting and decay of a thousand years. We choose to call these two principles antagonistic, because in their effects they are entirely opposite; yet there is no discordance, no caprice in their operation. Each works out its end, and the result is the harmony and stability of the face of nature.

In the world of life, too, there seems to have been a double principle of decline and renewal. The natural tendency of species and genera, like that of individuals, has been towards extinction. Why it should be so we know not, further than that they are for the most part influenced by every change in physical geography. But they probably obey a still higher law which governs their duration, as the laws of vitality govern the life of an individual If we are but slightly acquainted with the agency by which the degradation of land is counter-balanced, we are still more ignorant of the laws that preserve the balance of life. Creation is a mystery, and such it must for ever remain. So, too, are the principles on which it has been conducted. We can but mark their results. We see new species appear from time to time in the upward series of the geological formations, but they tell not whence they came. Of two genera created together at the beginning, one ere long died out,but the other still lives; yet here there is assuredly nought like discordance or caprice. Nay, these two principles—death and creation—have been in active operation all through the ages, and the result is that varied and exquisitely beautiful world wherein we dwell.

The Mollusca are so named from the soft nature of their bodies, and are familiar to us as exemplified in the garden-snail and the shells of the sea-shore. The general type upon which they are constructed is that of an external muscular bag, either entire or divided into two, called the mantle, in which the viscera are contained. In most of the orders, they have likewise an outer hard calcareous shell, consisting of one or more parts. It is of course this shell alone that can be detected in the rocks, but by attending to the relations between the living animals and their shells, we ascertain the nature and affinities of the fossil species.

Few who ramble by the sea-shore, gathering limpets, whelks, and cockles, are aware how complex an anatomy is concealed within one of those brown discoloured shells. There are elaborate nervous and muscular systems—sometimes several hearts with accompanying arteries and veins—often dozens of rudimentary eyes—capsules which perform the function of ears—jaws, teeth, a strongly armed tongue—gullet, gizzard, stomach, liver, intestine, and complete breathing apparatus. The structure and grouping of these parts vary in the different genera and orders, and upon such variations is founded the classification of the naturalist. Thus, the mollusca of the highest class are called theCephalopoda, orhead-footed, because their feet, or rather arms, are slung in a belt round the head. They contain, among their number, the cuttle-fish, with its curious internal bone that shadows forth, as it were, the coming of the vertebrate type; and the nautilus, with its many-decked vessel of pearl. The second class is termed theGastropoda, orbelly-footed, as the genera embraced under it creep on the under side of the body,which is expanded into a broad retractile foot. The common snail and whelk are familiar examples. The third class is formed by thePteropoda, orwing-footed—delicate animals, found only in the open sea, and remarkable for a pair of wing-like expansions or fins on the sides of the mouth. TheLamellibranchiataform the fourth class, and receive their name from the laminated form of their branchia, or gills. They contain the two-valved shells, such as the oyster and scallop, and are one of the most abundant groups of animals on our coasts. The fifth class consists of theBrachiopoda, orarm-footedmolluscs a name given to them from their long spiral arms, once thought to be the instruments of motion, but now ascertained only to assist in bringing the food to the mouth. The sixth, and humblest class, has received the designation ofTunicata, from the thick bladder-like tunic, or sac, which supplies the place of an outer shell.

The geologist finds the remains of all these classes in the different rock-formations of the crust of the earth. They flourished so abundantly in the earliest seas, that the first geological period has sometimes been called the Age of Molluscs; and, during all the subsequent eras, they held a prominent place among the inhabitants of the deep. Let us look for a little at their development in the times of the Carboniferous system.

As the Carboniferous group of rocks exhibits the remains of ocean-bed, lake-bottom, and land-surface, so we find in it shells of marine, fresh-water and (though rarely) terrestrial mollusca. The marine genera greatly predominate, just as the shells of the sea at the present day vastly outnumber those either of lakes or of the land. In England they occur chiefly in the lower part of the formation, giving a characteristic stamp to the deep series of beds known as the mountain limestone. There they are associated with the corals and stone-lilies already described—all productions of the sea. In Northumberland, however, andgenerally throughout Scotland, they occupy a somewhat different position. The great mountain limestone of central England gets split up into subdivisions as it proceeds northward, and beds of coal, full of land plants, become mingled with the ordinary marine strata. Sometimes we may find a group of brachiopods scattered over the macerated stem of a stigmaria; and the writer has himself collected a sigillaria in a limestone crowded with stone-lilies andproducti. But this intermingling is still further carried on in the upper part of the series. The coal-beds, with their underclays and stigmaria rootlets, evidently representing ancient vegetation with the soils on which it grew, are succeeded by beds of limestone, full of marine mollusca; and these, again, are erelong replaced by sandstones, shales, and ironstones, charged with land-plants and fresh-water shells. To this curious blending of very different organic remains, I shall have occasion to refer more at large in a subsequent chapter. I mention it now as a sort of apology for the dryness of details which it is necessary to give, in order to complete our picture of the carboniferous fauna, and to understand the principles upon which the ancient history of the earth is deciphered.

Fig. 21.

Of thePteropoda, we have, as yet, but one carboniferous genus, theconularia(Fig. 21). It was a slim delicate shell, in shape an oblong cone, having four sides, finely striated with a sort of zig-zag moulding like that of the Norman arch. Each of the four angles was traversed along its whole extent by a narrow gutter-like depression, and this style of fluting, combined with the markings on the sides, imparted no little elegance to the shell. The conularia is not a common fossil. It has been found among the coal-bearing strata of Coalbrook-Dale, and was noticed long ago by Dr. Ure in hisHistory of Rutherglen.

TheBrachiopodaare bivalve molluscs, but unlike most other molluscs they are rooted to one spot, and destitute of any power of locomotion. Their shells are unequal, the dorsal, or upper valve, being smaller and usually more bulged out than the under or ventral valve, which in most species is prolonged at its narrow end into a kind of beak. In the terebratula this beak has a little circular hole, from which there emerges a short peduncle or stalk, that fixes itself firmly to a rock or other substance at the sea-bottom, and serves the purpose of an anchor and cable to keep the little vessel safely moored. When the shells are detached, these perforated ventral valves have so exactly the form of the old Roman lamps, "that they were calledLampades, or lamp-shells, by the old naturalists."[36]Other species, as thelingulæ, have no beak, and the long peduncle passes out between the valves, which are of nearly equal size, and have been compared to the shape of a duck's bill. In yet another genus, thecrania, there is no peduncle, but the animal adheres by its lower valve, much like the oyster, and may often be seen clustered in groups on decayed sea urchins or other organisms, particularly in the chalk formation.

[36]See the excellentManual of Mollusca, by Woodward, p. 209.

The internal structure of these animals is singularly beautiful. The inner surface of each valve is lined with a soft membranous substance, called the pallial lobe, the margin of which is set round with stiff hair-like bristles, that prevent the ingress of any foreign body likely to interfere with the play of the delicate filaments of the arms. These two soft lobes are furnished with veins, and supply the place of a breathing apparatus. The body of the animal occupies not quite a third part of the interior of its valves, and is situated at the narrow end. There are thus two distinct regions within the shell, separated from each other by a strong membrane, through the centre of which is the opening of the mouth. The smaller cavity next the hinge contains the viscera, and the outer larger one, the folded and ciliated arms.These arms form one of the most characteristic features of the brachiopods. They are two in number, and proceeding from the margin of the mouth, advance into the outer empty chamber of the shell, and return upon themselves in spiral curves and folds. They are fringed with slim, flat, narrow filaments, set along the arm like teeth along the back of a fine comb. Though called arms, these long ciliated appendages are rather enormously protruded lips. The vibratory action of the fringes causes currents to set inwards towards the mouth, which is placed at the inner end or base of the arms. To support these long convoluted arms, many of the genera are furnished with slender hoops of hard calcareous matter, which are hung from the dorsal valve, and are still found within the shells of some of the most ancient fossil brachiopods.

The little visceral cavity contains the complex groups of muscles for opening and closing the valves, a simple stomach, a large granular liver, a short intestine, two hearts, and the centre of the nervous system. Without going into the details of these various structures, the reader will see that the brachiopoda are really a highly organized tribe; and I am thus particular in the enumeration, partly that he may the better understand the mechanism of the carboniferous shells of that type, and partly that he may mark how the oldest forms of life, those that meet us on the very threshold of animated existence, were not low in organization, but possessed an anatomy as complex as it was beautiful.

Who that has ever wielded an enthusiastic hammer among the richly fossiliferous beds of the mountain limestone, does not remember with delight the hosts of delicately fluted shells that the labour of an hour could pile up before him? There was the striated productus, with its slim spines scattered over the stone. There, too, lay the spirifer with its broader plications, its toothed margin, and its deeply indented valve. Less common, and so more highly prized, was the slimly-ribbed rhynconella,with its sharp, prominent beak, or perhaps the smooth, thin terebratula, with its colour-bands not yet effaced. These were pleasant hours, and their memory must dwell gratefully among the recollections of one whose avocations immure him throughout well-nigh the livelong year amid the din and dust of town—thefumum et opes strepitumque Romæ.

Fig. 22.—Productus giganteus.

In theproductusthe dorsal valve is sometimes quite flat, while the ventral is prominently arched, and the shell resembles a little cup with a flat plate of the same diameter placed over it. Usually, however, both the valves are concavo-convex, or arched in the same direction like two saucers placed within each other. The exterior surface of each valve is differently ornamented in the various species. A very common style of sculpturing is by a set of fine hair-like longitudinal ribs, diverging more or less regularly from the hinge line to the outer margin. In some species these ribs are wider, and are furnished with little prominent scars. In others (asP. punctatus) a set of semicircular ridges runs round the shell, narrowing as they converge from the outer lips to the centre of the hinge line, and bearing each an irregular row of small scars or tubercules. Some of the species are very irregularly ornamented into a sortof wrinkled surface, in which the striæ seem, as it were, thrown over the valves in bundles at random.

The productus was furnished with slender hollow spines, which rose up from the surface of either valve, chiefly, however, about the hinge. InP. spinosusthey were long and stout, like thin rush stalks, while in the smaller species they rather resembled stiff bristles. The use of these spines is not very well made out. As most of the producti appear to have been free, that is, without any peduncle fixing them to the sea-bottom, it has been conjectured that the spines, by sinking deep into the mud, may have served the place of a peduncle to moor the shell.

As regards size, the productus is very variable. You may gather some species in the young form, not larger than peas, while others may reward your search, having a breadth of six or eight inches (P. giganteus). But however much they may vary in dimensions, they usually remain pretty constant in their abundance, being among the most common fossils of the mountain limestone, and even of some limestones in the true Coal-measures;[37]and that must be a poor stratum indeed which cannot yield you a bagful of producti.

[37]See the table given below inChap. X.

The productus no longer ranks among living forms. It began during the times of the Upper Silurian system, lived all through the Old Red Sandstone, and attained its maximum of development in the seas of the Lower Carboniferous group. As the coal forests began to flourish, the productus seems to have waned; but it is still sometimes found in considerable numbers in the ironstones and limestones intercalated among the coal seams of northern England and central Scotland. In the period which succeeded the coal, that, namely, of the Permian, it seems to have died out altogether, at least no trace of its remains have as yet been detected in strata of a later age. But whilst it lived, the productus must have enjoyed a wide rangeof climate, for its valves have been found by thousands both in the old world and in the new. I have seen several that were brought from the hills of China, and they occur likewise in Thibet. Specimens have been brought, too, from the warm plains of Australia, and from the snows of Spitzbergen.

In looking over the fossils that lie grouped along beds of the mountain limestone, there are two forms that we find almost invariably side by side—the productus and the spirifer. They seem to have begun life together, or rather, perhaps, the spirifer is somewhat the older brother. They voyaged through the same seas, and anchored themselves to the same ocean-bed, sometimes among mud and ooze, and often among bowers of corals and stone-lilies. They visited together the most distant parts of the world, from China to Chili, and from Hudson's Bay to New Zealand. I have sometimes laid open fragments of limestone where they lay thickly clustered as though they had ended a life of friendship by dying very lovingly together. But after all the varieties of the productus had died out, some species of the spirifer still lived on, and it was not until the period of the lias that they finally disappeared. I remember meeting with one of these latest spirifers in the course of a ramble in early morning along the shores of Pabba, one of the lone sea-girt islands of the Hebrides, where the Scottish secondary rocks are represented. The beach was formed of low shelving reefs of a dark-brown micaceous shale, richly charged with the characteristic fossils of the Lias—ammonites, belemnites, gryphææ, pectines, &c. In the course of the walk I came to a lighter coloured band, with many reddish-brown nodules of ironstone, but with no observable fossils. A search, however, of a few minutes disclosed a weathered specimen, near which a limpet had made good its resting-place; and this solitary specimen proved to be one of the last lingering spirifers (S. Walcottii). The form struck me at once as a familiar one, and recalled the fossils of the mountain limestone. It mayseem a puerile fancy, but to one who had lately been working among palæozoic rocks, and remembered the history of the spirifer, there was something suggestive in the loneliness of the specimen. With the exception of one or two other organisms (asrhynconella), it was by far the most ancient form of the deposit. Its family had come into the world thousands of years before that of the large pinnæ that lay among the neighbouring shales, and perhaps millions of years before that of the gracefully curved ammonites. But the family was nearly extinct when these shales were being thrown down as sandy mud, and this wasted specimen, worn by the dash of the waves, seemed in its solitariness no inapt representative of an ancient genus that was passing away.

The spirifer received its name from the two highly developed spiral processes in the interior of the shell attached to the dorsal valve. They were hard, like the substance of the shell, and sprang from near the hinge, each diverging outwards to near the border of the valve. They resembled two cork-screws, but the loops were much closer together. These coiled calcareous wires almost filled the hollow of the shell (Fig. 23), and ample support was thus afforded to the filamentous arms. In recent brachiopods, these arms do not always strictly follow the course of the calcareous loops. Among palæozoic genera the case may have been similar, so that the complex calcareous coil of the spirifer may not perhaps indicate a corresponding complexity of the arms. But none of the few recent forms exhibit anything like the coiled processes of the spirifer.

The Carboniferous system of Great Britain and Ireland is stated to have yielded between fifty and sixty species of spirifers. Of course, in such a long list the gradations are sometimes very nice, and to an ordinary eye imperceptible, but there exist many marked differences notwithstanding. The general type of the spirifers is tolerably well defined. They had both valves arched outwards, not concavo-convex as in the productus.Their hinge-line, like that of the latter shell, ran in a straight line, and their dorsal valve was raised along its centre from hinge to outer margin, into a prominent ridge, while in the ventral valve there was a furrow exactly to correspond. Most of the species were traversed by sharp ribs radiating from the centre of the hinge-line like those on the surface of the common cockle. But some were quite smooth, retaining only the high lobe in the centre, such asS. glaber. In a noble specimen figured by M'Coy[38]under the name ofS. princeps, the valves are covered with broad plaits that sweep gracefully outward from the centre of the hinge-line.

[38]Carb. Limest. Foss. of Ireland, pl. 21, fig. 1.

Fig. 23.—Spirifer hystericus.b, Interior of the same, showing the arrangement of the spiral arms.

The spirifers vary more in form than in external ornament. Some are triangular, others nearly semicircular, others long and attenuated. In some species (as theS. glaber), the central ridge is very prominent, taking up about a third of the entire area of the shell, and thus giving it a trilobed appearance. In others (asS. symmetricus) it is less marked, and bears a minor furrow down its centre; while in yet a third class (as in some specimens ofS. trigonalis) the median fold scarcely rises above the ribs that are ranged on each side.

These old shells probably anchored themselves to the sea-bottom by means of a thin peduncle, and lived by the vigorousaction of those complex fringed arms, whose screw-like skeleton still occasionally remains, and which conveyed to the mouth the animal substances that served as food.

Fig. 24.—Terebratula hastata.

I shall refer to but one other brachiopod of the carboniferous rocks, interesting both as one of the forms of life still living in our seas, and as exhibiting, after the lapse of such a vast interval, the form of the coloured bands which adorned it when alive. It is calledTerebratula hastata; a slim delicate shell like its representatives of the present day, narrow at the beak, and bulging out towards the outer margin, which is slightly curved. The surface is smooth, and in the older specimens has numerous concentric layers of growth, especially marked near the margin. The stripes of colour radiate from the beak, outwards, and though the tint which once brightened them is no longer visible, it may be that the vessel of the little terebratula, which lay anchored perhaps fifty fathoms down, was well-nigh as gaily decked as a felucca of the Levant. But the existence of these colour-bands is not merely interesting; the geologist can turn it to account in investigating the physical conditions of an ancient ocean. The late Professor Edward Forbes, after a careful series of investigations in the Mediterranean, brought to light the fact, that below a depth of fifty fathoms shells are but dimly coloured, and hence he inferred, from the numerous coloured shells of the carboniferous limestone, that the ocean in which they lived was not much more than fifty fathoms deep.[39]

[39]Similar coloured bands are found even in the Lower Silurian, e.g., on turbo rupestris (Murchison'sSiluria, p. 194), while on many of the carboniferous gastropods and lamellibranchiate bivalves, they are of frequent occurrence.

Thelamellibranchiatebivalve shells of the British Carboniferous system, so far as yet discovered, number about 300species, belonging to genera some of which are still familiar to us. There were thepectensorscallops, thepinnaswith their beards of byssus, thecardiumsor cockles, themytiliandmodiolæor mussels, all sea-shells. Then among the fresh-water bivalves we can detect several species of the unio or river mussel, that perhaps displayed valves as silvery in their lining as those of our own pearl-mussels. But with these well-known forms there co-existed some that no longer survive. Such was theconocardium, a curious form that looks like acardiumcut through the middle, with a long slender tube added to the dismembered side (Fig. 25). Theaviculopecten, a shell allied to our common scallop, and sometimes showing still its colour-bands (Fig. 25), and thecardiniaoranthracosia, a small bivalve that abounds in the shales and ironstones of our coal-fields, along with nautili, producti, and conulariæ at Coalbrook Dale, and with a thin leaf-like lingula at Borrowstounness.

Fig. 25.—Carboniferous Lamellibranchs.1. Aviculopecten sublobatus (showing colour-bands). 2. Conocardium aliformis.

TheGastropodsof the carboniferous rocks in the British Islands embrace from twenty-five to thirty genera, with upwards of 200 species. Here, too, we can detect some forms that have not yet passed away. Thetrochus, so universally diffused over the globe at the present day, also lived in the palæozoic seas. Its companions, thenatica, theturritella, and theturbo, likewise flourished in these ancient waters. Among the genera now extinct we may notice theeuomphalus, with itswhorls coiled in a flat discoidal form; and thebellerophon, with its simple coiled shell, resembling in general form the nautilus. The gastropods are numerously represented in our gardens and woods, by the various species of the snails, animals that have a most extensive distribution over the world, and number probably not much under two thousand species.

Fig. 26.—Carboniferous Gastropods.1. Euomphalus peatangulatus. 2. Pleurotomaria carinata (showing colour-bands).

For a long time it was matter of surprise that no such land shells had ever been detected in the carboniferous rocks. Trees and forests had been turned up by the hundred, but never a trace was found of any air-breathing creature. From this fact, and from the enormous amount of vegetable matter preserved, it was once hastily inferred that the atmosphere of that ancient period must have been uncongenial to air-breathers; that, in short, it was a dense heated medium of noxious carbonic-acid gas, wrapt round the earth like a vast mephitic exhalation, favourable in the highest degree to the growth of vegetation, yet deadly as the air of Avernus to all terrestrial animals. But this notion, like most other bold deductions from merely negative evidence, has had to be abandoned, for traces of air-breathers have at last been found. Among these, not the least interesting is the shell of apupa, a sort of land-snail, which Sir Charles Lyell detected, along with the bones of a small reptile, embedded in the heart of an upright sigillaria stem in the carboniferousrocks of Nova Scotia. Small as was the organism, the evidence furnished by it proved scarcely less valuable than if it had been a large mammal that might have afforded material for weeks of study. The similarity of the shell to existing forms, showed that the ancient carboniferous forests had at least one race of air-breathing creatures among their foliage, and that the atmosphere of the period could have differed in no material point from that of the present day, for as the snails breathe by lungs, and require, consequently, a continual supply of oxygen to support respiration, they could not have existed in an atmosphere charged with carbonic acid.

Fig 27.—Carboniferous Cephalopods.1. Nautilus Koninckii. 2. Goniatites crenistria. 3. Orthoceras laterale (fragment).

TheCephalopods, or highest class of mollusca, are represented among the British carboniferous strata by seven genera. Of these the most characteristic is theorthoceras, 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 aseptum, through the centre of which a tube ran to the narrowend 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 thesyphon, 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 toone 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.

Thenautilus(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 thegoniatite(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—angledshell.

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 rareis 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.

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