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.
Holy George 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 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
characteristics of 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.
[Footnote 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's _Footprints
of the Creator_. See also M'Cosh on _Typical 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]
[Footnote 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]
[Footnote 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 temperature remains 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 of _rhynconella_ now 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.
[Footnote 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, after many
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 the _rhynconella_, 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. Hills are 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
the _Cephalopoda_, or _head-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 the
_Gastropoda_, or _belly-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 the _Pteropoda_, or _wing-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. The
_Lamellibranchiata_ form 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 the _Brachiopoda_, or _arm-footed_ molluscs 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 of
_Tunicata_, 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, and generally 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 and _producti_.
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.
[Illustration: Fig. 21.]
Of the _Pteropoda_, we have, as yet, but one carboniferous genus,
the _conularia_ (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 his _History of
Rutherglen_.
The _Brachiopoda_ are 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 called _Lampades_,
or lamp-shells, by the old naturalists."[36] Other species, as the
_lingulæ_, 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, the _crania_, 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.
[Footnote 36: See the excellent _Manual 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--the _fumum
et opes strepitumque Romæ_.
[Illustration: Fig. 22.--Productus giganteus.]
In the _productus_ the 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 (as _P. 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 sort of 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. In
_P. spinosus_ they 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.
[Footnote 37: See the table given below in Chap. 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 range of 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
may seem 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 (as _rhynconella_), 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 as
_S. glaber_. In a noble specimen figured by M'Coy[38] under the name
of _S. princeps_, the valves are covered with broad plaits that sweep
gracefully outward from the centre of the hinge-line.
[Footnote 38: _Carb. Limest. Foss. of Ireland_, pl. 21, fig. 1.]
[Illustration: 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 the _S. 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 (as _S. symmetricus_) it is
less marked, and bears a minor furrow down its centre; while in yet a
third class (as in some specimens of _S. 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 vigorous action 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.
[Illustration: 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 called
_Terebratula 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]
[Footnote 39: Similar coloured bands are found even in the Lower
Silurian, e.g., on turbo rupestris (Murchison's _Siluria_, p. 194), while
on many of the carboniferous gastropods and lamellibranchiate bivalves,
they are of frequent occurrence.]
The _lamellibranchiate_ bivalve shells of the British Carboniferous
system, so far as yet discovered, number about 300 species, belonging
to genera some of which are still familiar to us. There were the
_pectens_ or _scallops_, the _pinnas_ with their beards of byssus,
the _cardiums_ or cockles, the _mytili_ and _modiolæ_ or 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 the _conocardium_, a curious form that looks like a
_cardium_ cut through the middle, with a long slender tube added to
the dismembered side (Fig. 25). The _aviculopecten_, a shell allied to
our common scallop, and sometimes showing still its colour-bands (Fig.
25), and the _cardinia_ or _anthracosia_, 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.
[Illustration: Fig. 25.--Carboniferous Lamellibranchs.
1. Aviculopecten sublobatus (showing colour-bands). 2. Conocardium
aliformis.]
The _Gastropods_ of 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. The _trochus_, so universally diffused over the globe at the
present day, also lived in the palæozoic seas. Its companions, the
_natica_, the _turritella_, and the _turbo_, likewise flourished in
these ancient waters. Among the genera now extinct we may notice the
_euomphalus_, with its whorls coiled in a flat discoidal form; and the
_bellerophon_, 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.
[Illustration: 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 a _pupa_, 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 carboniferous rocks 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.
[Illustration: 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.
[Footnote 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.]