Classification of the naturalist not always correspondent with
the order of nature--Incongruous grouping of animals
in the invertebrate division--Rudimentary skeleton
of the cephalopods--Introduction of the vertebrate
type into creation--Ichthyolites of the carboniferous
rocks--Their state of keeping--Classification of fossil
fishes--Placoids--Ichthyodorulites--Ganoids--Their structure
exemplified in the megalichthys and holoptychius--Cranium of
megalichthys--Its armature of scales--Microscopic structure of
a scale--Skeleton of megalichthys--History of the discovery
of the holoptychius--Confounded with megalichthys--External
ornament of holoptychius--Its jaws and teeth--Microscopic
structure of the teeth--Paucity of terrestrial fauna in coal
measures--Insect remains--Relics of reptiles--Concluding
summary of the characters of the carboniferous fauna--Results.
The organic remains hitherto described belong to that large division
of the animal kingdom instituted by Lamarck, to comprehend all those
whose internal structure is supported by no vertebral column, and
which are hence termed invertebrate. They are for the most part
protected by a hard outer covering, or exo-skeleton, which assumes
many different modifications. We have seen it in the calcareous
cells of the little net-like fenestella, in the geometric cup of
the stone-lily, in the double case of the cypris, and in the shells
of the mollusca. But the order of nature does not always exactly
correspond with the classification of the naturalist. His system
must necessarily be precise, formal, and defined. One tribe ends off
abruptly, and is immediately succeeded by another, with different
functions and structure, and dignified with a separate name. But in
the order of creation, such abrupt demarcations are few, for if they
exist in the present economy, they can not unfrequently be filled up
from the existences of the past. There is usually a shading off of
one class into another, like the blending of the tints of sunset,
and it often baffles all the skill of the profoundest anatomist, by
drawing a distinct line, to pronounce where the one division actually
ends and the other begins. Any name, therefore, which is intended to
embrace a large section of the animal kingdom, must ever be more or
less arbitrary. It will extend too far in one direction, and embrace
organisms which might be classed in a different section. It will
probably not extend far enough in another, and thus leave beyond its
pale animals possessing strong affinities to the majority of those
included under it. More especially is this true of every system of
classification that proceeds upon the modifications of a single
feature, or upon mere negative resemblances. Suppose, for instance,
that it were proposed by some highly systematic individual to divide
the inhabitants of our country into two great classes--the bearded
and the beardless. In the latter category he would arrange all the
more quiet and orderly portion of the community, with perchance a
tolerable intermixture of rogues. The bearded group would present a
most motley array--from the fierce-visaged heroes of the Crimea to the
peaceable stone-mason or begrimed pitman--all brought into one list,
and yet agreeing in no single feature save that of being like Bully
Bottom the weaver, "marvellous hairy about the face." But Lamarck's
invertebrate division of the animal kingdom presents a grouping of yet
more diverse characteristics, as cannot fail to be confessed when we
recollect that it embraces among its members the microscopic monad,
the coral polyp, the lobster, the butterfly, the limpet, the nautilus,
and the cuttle-fish. Cuvier's three-fold grouping of the division
into _mollusca_, _articulata_, and _radiata_, has now supplanted the
old name, though the latter is still retained as a sort of convenient
designation for all the animals below the vertebrate type.
The most highly developed of the recent cephalopods exhibit a true
internal skeleton, in the form of a strong oblong bone, on which the
body is hung. In this respect they occupy a sort of intermediate place
between the lower molluscs on the one hand, and the lower fishes on
the other. Theirs is not a vertebral column, but rather, as it were,
a foreshadowing of it; not, however, as a link in some process of
self-development from mollusc to fish, for these higher cephalopods do
not appear to have been created until fishes and reptiles had lived
for ages. The vertebrate type has been traced well-nigh as far back
into the past as we have yet been able to penetrate. Once introduced,
it has never ceased to exist, but in the successive geological ages
has been ever receiving newer and higher modifications, reaching its
perfection at length in man. The vertebrate form of structure fulfils
the highest adaptations of which terrestrial beings seem capable. We
can hardly conceive of corporeal existence reaching a more elevated
stage of development, save in thereby becoming less material, and
receiving an impartation of some higher element. The vertebrate
animals display not merely the most complexly organized structures,
but manifest in their habits the workings of the higher instincts
and affections. Among the invertebrate tribes the propagation of the
species is, in the vast majority of cases, a mere mechanical function,
like that of feeding or respiration, and the eggs once deposited, the
parent has no further care of her young. But among the vertebrated
animals, on the other hand, the perpetuation of the race forms the
central pillar round which the natural affections are entwined. It
parcels out every species into pairs, in each of which the mates are
bound together by the strongest ties of attachment. It gives birth,
too, to that noble instinct which leads the mother to expose her own
life rather than suffer harm to come to her offspring. It produces, at
least in man, that reciprocal attachment of offspring to parent, from
which springs no small part of all that is holiest and best in this
world. These attributes, to a greater or less extent, belong to all
the vertebrate animals, from the fish up to man. In looking over the
relics of animal life in the earlier geological formations, we are apt,
as we gaze on the massive jaws and teeth, the strong bony armour, and
the sharp, barbed spines, to think only of a time of war and carnage,
when the larger forms preyed upon the smaller, or ruthlessly sought
to exterminate each other. Yet should we not remember, that with all
these weapons and instincts of self-preservation there were linked
attributes of a nobler kind; that the earliest vertebrate remains point
to the introduction--though perhaps in but a rudimentary form--of
self-sacrificing love into our planet? The march of creation from
the first dawn of life has ever been an onward one, as regards the
development not only of organic structure but of the social relations;
and if it be true that physical organization finds its archetype in
man, it is assuredly no less so that in him too we meet with the
highest manifestation of those instincts which, by linking individual
to individual, have ever marked out the vertebrate tribes of animals
from the more machine-like characteristics of the invertebrate.
We pass now to the vertebrate animals, and shall look for a little into
the general grade and organization of the fishes that characterized the
carboniferous rivers and seas.
A collection of the ichthyolites of the carboniferous rocks presents
almost every variety in the mode of preservation. The smaller species
are frequently found entire, and show their shining scales still
regularly imbricated as when the creatures were alive. The larger forms
seldom occur in other than a very fragmentary condition. The limestones
yield dark-brown or black, oblong, leech-like teeth, which are found
on examination to be those of an ancient family of sharks. The shales
are often sprinkled over with glittering scales and enamelled bones.
Some of the coals and ironstones yield in abundance long sculptured
spines, huge jaws bristling with sharp conical teeth, and detached
tusks, sometimes five or six inches long. In short, the naturalist who
would decipher the ichthyology of the Coal formation, finds before
him, in the rocks, not a suite of correctly arranged, and carefully
preserved skeletons, but a set of disjointed, unconnected bones; here
a tooth, there a scale, now a jaw, now a dermal plate, all mingled at
random. And yet, though the evidence lie in this fragmentary state, our
knowledge of these ancient fishes is far from being correspondingly
meagre. To such precision has the science of comparative anatomy
arrived, that a mere scale or tooth is often enough to indicate the
nature and functions of the individual to which it belonged, and to
establish the existence in former times of a particular class or order
of animals. Thus the smooth rounded teeth of the mountain limestone are
found to present both externally and internally a close resemblance to
the hinder flat teeth of the sole living cestraciont (_C. Philippi_);
and we hence learn that a family of sharks, now all but extinct,
abounded in the palæozoic seas. The occurrence of a set of dark,
rounded little objects, which by the unpractised eye would be apt to be
mistaken for pebbles, is in this way sufficient at once to augment our
knowledge of the various animals of the Carboniferous period, and to
establish an important fact in the history of creation.
Of the four great Orders into which Agassiz[41] subdivided the class
_Pisces_, the Placoids and Ganoids, agreeing on the whole with the
cartilaginous fishes of Cuvier, occur abundantly in the palæozoic
rocks, while the Cycloids and Ctenoids, answering to Cuvier's
osseous fishes, began in the Secondary formations, and are found in
all subsequent deposits. The two former reached their maximum in the
earlier geological ages, and have been gradually dwindling down ever
since, till now they are represented by comparatively few genera; the
two latter are emphatically modern orders; they have been constantly
increasing in numbers since their creation, and swarm in every sea at
the present day. The carboniferous ichthyolites belong, of course, only
to the two first-mentioned orders the placoids and ganoids.
[Footnote 41: The classification of Agassiz, which is certainly not
a little arbitrary and artificial, has been altered by Müller, a
distinguished German anatomist, whose arrangement has been modified
again by Professor Owen. See Owen's _Lectures on Comparative Anatomy_,
vol. ii. p. 47. There is far from anything like unanimity on the
subject. Every naturalist thinks himself at liberty to modify and
restrict the groupings of his predecessors or contemporaries, sometimes
without condescending to give synonyms or any clue by which one may
compare the rival classifications. The geological student cannot engage
in a more sickening task than that of ranging through these various
arrangements, and he must possess some self-command who can refrain
from throwing up the search in disgust. The best way of progressing is
to select some standard work and keep to it, until the characteristics
of the genera and families have been mastered, and as far as possible,
verified from actual observation. After such preliminary training, the
student will be more able to grope his way through the "chaos and dark
night" of synonyms and systems.]
The Placoid, or _Plagiostome_ fishes, are familiar to us all as
exemplified in the common thornback and skate of our markets. They are
covered with a tough skin, which either supports a set of tuberculed
plates as in the thornback, or a thick crop of small rounded bony
points or plates, as in the shagreen of the sharks. The head consists
of a single cartilaginous box. The spinal column is likewise formed
of cartilage, built up in the higher genera of partially ossified
vertebræ. The tail is heterocercal or unequally lobed, inasmuch as
the spinal column, instead of ending off abruptly as it does in the
herring, trout, and all our commoner fishes, passes on to the extreme
point of the upper half of the tail. This is a noticeable feature, for
it has been found to characterize all the fishes that lived in the
earlier geological periods. The fins are often strengthened by strong
spines of bone, which stand up in front of them and serve the double
purpose of organs of progression and weapons of defence. The teeth vary
a good deal in form. In the larger number of existing placoids they
are of a sharp cutting shape, often with saw-like edges. Among the
sharks they run along the jaws in numerous rows, of which, however,
only the outer one is used, those behind lying in reserve to fill up
the successive gaps in the front rank. The teeth do not sink into the
jaw, as in the ganoids, but are merely bound together by the tough
integument which forms the lips. Another form of tooth, abundant among
the ancient placoids, and visible on some of those at the present
day, shows a smooth rounded surface, the teeth being closely grouped
together into a sort of tessellated pavement which, in the recent
species, runs round the inner part of the jaws, while a row of conical
teeth guards the entrance of the mouth.
[Illustration: Fig. 28.--Ctenacanthus hybodoides. (Edgerton.)]
The animals which possess these characteristics include the various
tribes of the sharks and rays, and form the highest group of fishes.
They are all active and predaceous, frequenting every part of the ocean
where their prey is to be found. The formidable spines and hideous
"chasm of teeth" belonging to the bulkier forms, render them more than
a match for any other denizens of the deep, and thus they reign in
undisputed supremacy--the scourge of their congeners, and a terror to
man.
The seas of the Carboniferous era abounded with similar predaceous
fishes, some of which must have been of enormous size. An entire
specimen has never been obtained; nor, from the destructible nature
of the animal framework, can we expect to meet with one. But the hard
bony parts of the animals, those capable in short of preservation
in mineral accumulations, are of common occurrence in the mountain
limestone beds and even among the coal seams. The dorsal spines or
_ichthyodorulites_, are especially conspicuous (Fig. 28). They stood
up along the creature's back like masts, the fin which was attached to
the hinder margin of each, representing the sail. The spine could be
raised or depressed at pleasure, its movements regulating those of the
fin, much as the raising or lowering of the mast in a boat influences
the lug-sail that is attached to it. The general form of these spines
was long, tapering, and more or less rounded. But they assumed many
varieties of surface ornament. Some species were ribbed longitudinally,
and had along their posterior concave side a set of little hooks
somewhat like the thorns of a rose. Others seem to have been quite
smooth, and of a flattened shape, with a thick-set row of sharp hooks
down both of the edges, like the spine on the tail of the sting-ray
of the Mediterranean. Such weapons have considerable resemblance to
the barbed spear-heads of savage tribes, and it is certain they were
intended to act in a similar way, as at once offensive and defensive
arms. The toothed spines of the sting-rays are still used in some
parts of the world to point the warrior's spear and arrow. Is there
not something suggestive in the fact that these stings, after having
accomplished their appointed purpose as weapons of war in the great
deep, should come to be employed over again in a like capacity on the
land; and that an instrument, which was designed by the Creator as a
means of protecting its possessor, should be turned by man into an
implement for gratifying his cupidity and satiating his revenge? Other
ichthyodorulites are elegantly ornamented by long rows of tuberculed
lines arranged in a zig-zag fashion, or in straight rows tapering from
base to point. In all there was a blunt unornamented base, which sank
into the back and served as a point of attachment for the muscles
employed in raising or depressing the spine. In some specimens the
outer point appears rounded and worn, the characteristic ornament being
effaced for some distance--a circumstance which probably indicates that
these fishes frequented the more rocky parts of the sea.[42]
[Footnote 42: See Egerton, _Quart. Jour. Geol. Soc._, vol. ix. p. 281.]
The placoid teeth of the carboniferous rocks show the usual forms
of the order. Some of them are sharp and pointed, as those of the
hybodonts; others have a smooth, rounded, or plate-like form, as in the
cestracionts. The latter often show a dark brilliant surface, and might
be readily enough mistaken for well-worn pebbles. In the oblong rounded
teeth of _psammodus_ the surface is densely covered with minute points
like grains of sand, whence the name of the genus. These teeth, when
sliced and viewed under the microscope by transmitted light, exhibit a
complex reticulated internal structure.
Agassiz' second great Order of fishes is named Ganoid, from a Greek
word signifying brightness, in allusion to the brightly enamelled
surface of their dermal covering. They differ from the placoids in
having their outer surface cased in a strong armature of bone, which
is disposed either in the form of large overlapping plates, as among
the strange tortoise-like fishes of the Old Red Sandstone, or as thick
scales, which are either placed at intervals, as along the back and
sides of the sturgeon, or closely imbricated, as in the stony-gar
(_lepidosteus_) of the American rivers. This strong, massive skeleton
constitutes in many genera the sole support of the animal framework,
the inner skeleton being of a gristly cartilaginous kind, like that
of the skate. On this account traces of the vertebral column are by
no means abundant among the older formations. But as the ganoids form
a sort of intermediate link between the placoid or gristly fishes
on the one hand, and the bony fishes on the other, they are found
to present in their different genera examples of both these kinds
of structure. Thus, the skeleton of the sturgeon consists of a firm
cartilage, out of which the vertebræ are moulded, so that this fish
was at one time ranked with the sharks in the cartilaginous tribe of
Cuvier. The skeletons of some of the older ganoids (as _holoptychius_),
on the other hand, manifest such a decidedly osseous structure, with
sometimes so much of a reptilian cast, that the bones were at first
referred to some huge extinct saurians. The head of the ganoid fishes
is encased in a set of large massive plates of bone, and the jaws
are furnished with several rows of small sharp teeth, intermingled
with a less numerous but larger-sized and more formidable kind. The
interior of the mouth likewise displayed in many ancient genera groups
of palatal teeth, so that the dental apparatus of these animals
must have been very complex and complete. The tail in all the older
ganoids was heterocercal, like that of the sharks, the lobes being
not unfrequently densely covered with minute overlapping scales of
bone--a peculiarity which also extended to the fins. But the fins
were sometimes strengthened in another way by having the foremost ray
greatly thickened and enlarged, so as to form a stiff spine like the
ichthyodorulites of the placoids. The whole of the external surface
of these ganoidal fishes glittered with enamel, and was usually
sculptured in the most graceful patterns or ornamented with fine
lines and punctures so minute as to be almost invisible to the naked
eye. Every plate, scale, fin-ray, nay, the very lips exhibited the
characteristic enamel mottled over with the style of ornament peculiar
to the species. And when we think we have exhausted the contemplation
of these beauties, it needs but a glance through an ordinary microscope
to assure us that the unassisted eye catches only a superficial glimpse
of them. The more highly we magnify any portion of these old-world
mummies, the more exquisite does its structure appear.
In the carboniferous rocks of Great Britain, upwards of forty species
of ganoids have been detected. They have a wide range in size, the
smallest measuring scarce two or three inches, while the largest, to
judge at least from the bones which they have left behind, must have
reached a length of twenty, or perhaps even thirty feet The lesser
genera (Fig. 29) were characterized by small, angular, glossy scales,
usually ornamented either with a very minute punctulation, or with
fine hair-like lines which sometimes exhibited the most complicated
patterns. The scales were likewise occasionally serrated along the
exposed edges--a style of ornament which gives no little richness to
the aspect of the dermal covering. The fins, closely imbricated with
small angular scales of bone, sometimes displayed a striated ray in
front, but this neither possessed the strength nor the formidable
aspect of the corresponding spine among the placoids. The head was
encased in a set of bony plates fitting tightly into each other, and
ornamented with various patterns according to the species. The teeth
were very small and fine, resembling the bristles of a brush, but in
at least some species intermingled with teeth of a larger size. The
minute style of dentition in these smaller fishes has been thought to
indicate their habit of keeping to the bottom of the water and feeding
on the soft decaying substances lying there. Nowhere have I seen the
small rhomboidal scales of the _palæoniscus_ so abundant as among dark
shales charged with cypris cases and fragments of terrestrial plants,
and on such occasions the idea has often occurred that these graceful
little fishes, like the _amia_ of the American rivers, may have fed on
the cyprides that swarmed along the bottom of the estuary.
[Illustration: Fig. 29.--Amblypterus macropterus (a Carboniferous
ganoid).]
Scattered over the fresh-water limestones, ironstones, and shales, or
crowded together along the upper surface of some of the coal-seams,
there occur the remains of two very remarkable ganoidal fishes. They
deserve our attention for their great size, their complex organization,
and the important place in the scale of animal life which they occupied
during a former period. One of them has been called _megalichthys_
or _great fish_--an unhappy name, since the animal did not reach the
dimensions attained by not a few of the other ganoids, and was even
surpassed by at least one of its contemporary congeners. The other
is known as the _holoptychius_ or _wrinkled scale_. A more detailed
examination of these two animals will perhaps best enable us to
understand the character of the ganoid fishes that lived in the waters
of the Carboniferous period.
The megalichthys had an average length of about three feet. Like
the other members of the ganoid order it had a glittering exterior,
every scale and plate being formed of strong bone, and coated with a
bright layer of enamel. Wherever this polished surface extends, it is
found to be ornamented with a minute punctulation, the pores of which
lie thickly together like the finer dots of a stippled engraving.
The cranial plates are further varied by a scattered and irregular
series of larger punctures that look as if they had been formed by the
insertion of a pin-point into a soft yielding surface. The examination
of the head of the megalichthys as depicted in Fig. 30, will convey an
adequate conception of the structure of a ganoidal cranium.
[Illustration: Fig. 30.--Head of Megalichthys Hibberti, one-sixth of
natural size (Agass. _Poiss. Foss._ Tab. 63).
A Upper side. B. Under side. C Profile.]
The snout is formed of an elegantly curved bone (_c_) fringed along
its under edge with minute thick-set teeth. On either side it is
flanked by two triangular plates, which occupy the space between the
intermaxillary bone (_c_) and the upper jaws (_q q_). The eye orbits
seem to have been at the corners of the intermaxillary, circumscribed
by the sub-orbitals (_f g h_) and the ethmoids (_b_). The massive
intermaxillary bone had its posterior margin of an angular form, and
into the notch thus formed there was wedged the anterior end of a long
strip of plates, which expanded as they approached the occipital part
of the cranium, and terminated in three irregular plates that may
represent the place of the parietal and occipital bones. The space
between this belt and the upper jaws was occupied by three large plates
(_i k l_) which in other ganoids, as the _osteolepis_ of the Old Red
Sandstone, were united into a single pre-opercular bone of considerable
size. The operculum or gill cover (_m_) was relatively large, and
had an elegantly curved anterior margin. The upper jaws (_q_) were
comparatively small, and had a fringe of small conical teeth. The under
jaws (_r r_) reached to nearly double the length of the upper, and were
similarly set round with teeth. The teeth of the megalichthys, like
those of the living lepidosteus, consisted of two kinds, of which the
one bristled thickly along the outer edge of the jaw as sharp minute
points, averaging about a line in length, while behind this outer row
lay a scattered series of much larger teeth that sometimes rose nearly
an inch above the jaw. The external surface of these more formidable
tusks is smooth, glittering, and minutely striated with fine lines from
base to point, while the root of each is farther marked by a circle of
short, deep, longitudinal furrows. The internal structure displays a
close ivory, which when viewed under a microscope is seen to be made up
of fine tubes radiating from the outer surface to the hollow central
cavity. Some of the bones in the interior of the mouth seem to have
been also furnished with an apparatus of teeth. The under surface of
the cranium between the arch of the under jaws consists of two oblong
central plates (_t_) surrounded by a row of sixteen irregular ones,
eight on each side, and terminated in front by a large lozenge-shaped
scale (_u_) which fits into their angle of junction on the one side,
and into the symphysis of the jaws on the other. In the osteolepis
there were likewise two large plates terminating in a similar
lozenge-shaped one, but without the flanking rows. In the famous Old
Red holoptychius of Clashbennie, the under surface of the head had
but two plates, and in the still older and more gigantic asterolepis,
there was but one. It is the delightful task of the paleontologist
to compare and contrast these various pieces of mechanism, to mark
how what seems lacking in one comes to be supplied in another, and to
trace out the various modes in which, during the ages of the past,
Nature has wrought out the same leading plan, sounding, as it were, an
ever-changing series of modulations upon one key-note. In comparing
together the ganoids of the Old Red Sandstone and the Carboniferous
rocks, he finds that in the asterolepis--a fish belonging to the lower
part of the former formation--the pointed arch formed by the sweep of
the lower jaws is filled up by a single plate like some abbey-window
with its mullions knocked away, and built up with rude stone and lime.
Higher in the same group of rocks he meets with the cranium of the
holoptychius, where there is one straight central mullion running
in an unbroken line from the angle of the arch to its base. In the
osteolepis[43] he sees this mullion branching into two at its upper
end, so that the window consists of three divisions, as in the simplest
style of Gothic. Passing upwards into the Carboniferous system, he
encounters a still more ornate arrangement in the cranium of the
megalichthys. The central mullion with its two upper branches still
remains, but it is flanked by an additional one on each side, from
which there spring six cross bars that diverge obliquely with a slight
curve, so as to join the outer arch and subdivide the window into
nineteen compartments. So varied are the plans of the Divine Architect
in what to man may seem such a little matter as the piecing together of
a fish's skull.
[Footnote 43: Hugh Miller's _Footprints of the Creator_, p. 91.]
The body of the megalichthys was cased in an armature of as solid and
glittering bone as that which defended its head. Where the plates of
the cranium ended off they were succeeded by large rhomboidal scales
that crossed the body obliquely, and overlapped each other like the
metal plates in the antique scale-armour. Each scale consisted of
two parts, of which one had a rhomboid form and was covered over with
enamel, while the other ran round the two inner sides of the rhomb
as a broad unenamelled selvage deeply indented along its centre. It
was the enamelled portion alone that formed the outer surface, the
rough unpolished border being covered by the overlapping edges of
the adjoining scales. The scales had not a uniform thickness, but
were strongest at the covered part from which each thinned off to
the outer edges. In this way the thin edge of one scale pressed down
on the thick part of the subjacent one, and a covering of uniform
strength and smoothness was produced. Looking at a set of these scales
as they still occupy their original position on the creature's body,
it is scarcely more than a half of each which meets our eye; for the
unenamelled border occupied about a third of the entire surface, and
a fourth of the remainder was covered by the overlapping scales. The
effect of this arrangement must have been to combine great strength
with the most perfect flexibility. Notwithstanding the bulk of his
helmet and the weight of his scale-armour, we cannot conceive the
megalichthys to have been other than a lithe, active, predaceous fish,
dealing death and destruction among the herring-like shoals of little
palæonisci and amblypteri, though able to maintain perhaps but a
doubtful warfare with his more bulky contemporary, the holoptychius.
The internal structure of the scales of the megalichthys exhibits the
same provision for combining strength with the least possible amount
of material. Viewed in a transverse section under a magnifying power
of about eight diameters, they are seen to consist of three layers of
bone; each possessing a peculiar structure. The outermost is formed
of a tessellated pavement of minute round ocelli, having a fine brown
colour, and placed close together with considerable regularity. They
somewhat resemble little wheels, the axle being either a dark solid
nucleus or a small circular aperture, whence there radiates to the
outer rim a set of exceedingly minute fibres which were originally
hollow, and served as canals to carry on the growth of the scale. The
vacant space left where four wheels impinge on each other, forms one
of the pores that cover the enamelled surface of the scale. The whole
structure of this outer layer very closely resembles that presented by
the internal part of the base of the teeth, save that the confluent
lobes shown in the teeth become in the scale detached into separate and
independent circles. The central stratum of each scale is composed of
a loose open network of cancellated bone that passes into the layer on
either side, and resembles in its general texture the osseous vertebræ
of the same fish. The under layer, one end of which rested immediately
on the skin, approaches more to the firmness and solidity of the outer
one, but, in place of a tessellated, ivory-like pavement, it had a
close fibrous texture, with here and there a scattered cavity, and the
fibres were matted together so as to resemble the more solid structure
of the cranial bones. The effect of this triple arrangement must have
been to impart great strength and lightness to the external armature
of the fish; the middle spongy layer serving, by its porosity, at once
to deaden the effect of any blow aimed at the outside, and to give
buoyancy and lightness to what would otherwise have been a coat of
mail well-nigh, as ponderous as that of a feudal chief. One can hardly
conceive any implement of warfare in use among the lower animals of
strength enough to pierce this massive covering. But we shall find
as we go on that if the megalichthys had a strong defensive armour,
a bulkier neighbour had a still stronger offensive one, and that the
enamelled plates of the one fish were scarcely a match for the huge
pointed tusks of the other.
The megalichthys had an osseous skeleton, with vertebræ of a discoidal
form. These internal bones when viewed under the microscope are found
to display an open cancellated structure, resembling that of the
central layer in the scales. It thus appears that this ancient fish
was not merely defended by a hard external armour, but possessed an
equally solid framework of bone within.
Mingled with the scales and bones of the megalichthys, there are found
the remains of a still larger fish, to which the name of Holoptychius
has been given. Its external ornament differed entirely from that of
the animal last described. It possessed teeth sometimes six or seven
times larger, and jaws, plates, and bones of a form and dimensions
totally distinct. Strange as it may seem, however, these two fishes
have been constantly and systematically confounded from the time when
they were first discovered. Two or three years ago, there might be
seen in the British Museum several specimens of the holoptychius,
of which some bore the correct name, while the rest were labelled
"Megalichthys;" and a similar error prevailed in several of the other
museums.[44] The confusion can be traced very distinctly in the memoir
of Dr. Hibbert, who for the first time described the remains of these
fishes, and wrote according to information received from Agassiz.
[Footnote 44: The mistake was noticed in 1845 by Hugh Miller, who, in a
foot-note to his _First Impressions of England and its People_, p. 71,
well defines the distinctions between the two ichthyolites.]
In the autumn of 1832, the attention of the scientific public of
Edinburgh was directed to the extraordinary character of some fossil
remains obtained from the lime-quarries of Burdiehouse, a village about
four miles to the south of the town. Dr. Hibbert visited the locality,
and soon saw enough to excite his lively interest in its thorough
investigation. The Royal Society of Edinburgh warmly supported his
exertions, and by their means a large suite of specimens was eventually
obtained, which the Doctor from time to time described as they were
successively received. At the meeting[45] of the British Association in
Edinburgh, in 1834, the specimens were exhibited before the Geological
Section, and a memoir upon them read by their successful discoverer.
On the conclusion of the paper, a lively discussion ensued upon the
nature of the animal to which the scales and teeth had belonged.
Dr. Hibbert argued, from the deeply-furrowed teeth, and the strong,
massive cranial plates, that the animal must have been a reptile, and
supported his assertion by no small amount of anatomical skill. In the
midst of the discussion, a message was sent to the great ichthyologist
of Neufchatel, who happened to be at that time busily engaged in the
Zoological Section. Passing over the fossils as they lay grouped
upon the table, with that quick perception for which he is so justly
celebrated, Agassiz at once decided that the bones must have been
those of some large and hitherto undescribed _fish_. Such a decision
from such an authority produced of course no little sensation, and the
naturalist was told with some surprise that the remains had just been
elaborately described as those of extinct reptiles. "Reptiles!" thought
Agassiz, and again his quick eye darted over the table; but the fossils
would yield no other answer than what they had already given. Despite
their seeming reptilian character, they were undoubtedly ichthyic,
though belonging to an animal up to that time unknown. In the completed
memoir which Dr. Hibbert subsequently submitted to the Royal Society,
his mistake was freely acknowledged, and the remains there flourish as
those of a true fish. But with this amendment a grave error of another
kind was committed, though in this the Doctor seems to have been
supported by the authority of Agassiz himself. The large bones, scales,
and teeth of the Burdiehouse limestone, were all indiscriminately
thrown into one genus, to which Agassiz gave the name of Megalichthys;
and in the memoir we find the different kinds of scales and teeth
described and figured without the slightest intimation or suspicion
that they might possibly have belonged to different animals. The
novelty of the discoveries soon attracted general attention to
Dr. Hibbert's paper. It was quoted or referred to in almost every
scientific work treating of general geology, while in some instances
(as in Dr. Buckland's _Bridgewater Treatise_) the erroneously-named
bones were re-engraved. A tooth from the Fife coal-field, drawn
for one of the woodcuts in a popular elementary manual, was also
named megalichthys; an error perpetuated through every edition till
the last, where the tooth has been restored to its true owner--the
holoptychius. In truth, no two organisms have ever been so maltreated;
and if the reader will kindly bear with me a little further, it will
not be difficult to show him that the holoptychius had peculiarities
of its own quite as distinct as those that have come before us in the
megalichthys, and that each animal has a full and legitimate claim to a
separate and independent niche in the gallery of fossil fishes.
[Footnote 45: See Agassiz, _Poiss. Foss._, tom. ii. Part 2, p. 89 _et
seq._]
The word _holoptychius_ means, as I have said, "wrinkled or folded
all over,"--a name truly expressive of the peculiar style of ornament
displayed by every part of the exterior of the animal's body. The
head-plates, which are of great size, exhibit a fine corrugated
shagreen-like surface, roughened into knobs, and wavy lines of
confluent tubercules, that remind one disposed to be fanciful, of a
frosty December moon with its isolated peaks, and confluent mountain
chains. The scales are of a rounded or oval form, and vary from less
than half an inch to fully four or even five inches in diameter.
Their upper side consists of two parts, one of which with a crescent
shape lay beneath, the over-lapping scales, while the other passed
outwards to form a portion of the outer visible surface. The part
that was hidden by the overlapping scale was smooth, with a finely
striated surface. The exposed portion displayed the usual corrugated
sculpturing, many of the little tubercules having striated sides, and
showing, in consequence, no little resemblance to the star-like knobs
on the dermal covering of the Old Red Sandstone asterolepis. The inner
surface of the scales was concave, with a central prominent oblong
point surrounded by encircling scaly ridges, and forming what is called
the centre of ossification.[46]
[Footnote 46: The above descriptions of the scales and teeth of these
two fishes, are taken from specimens in my own collection. None of my
holoptychian scales show incontestable the proportion of the covered to
the exposed part. Judging from the aspect of one of them, the wrinkled
portion occupied perhaps about three-fifths of the entire scale,
the remaining part being covered by the overlapping edges of those
adjacent; for the characteristic corrugated surface was essentially
an external ornament, and ceased at the point where the external
bone passed into the interior. I may remark, that the upper side of
the scales is not very frequently seen in the Burdiehouse limestone,
the rough surface usually adhering to the rock, and leaving only the
smooth inner side exposed. Out of seven specimens from that locality,
only one shows the upper side, and that by no means in a perfect state
of keeping. The structure alike of scales and bones can be seen to
much greater advantage in the shales, ironstones, and coals of the
coal-fields, where, owing to the soft nature of their matrix, the
fossils can be readily cleared and exposed.]
But perhaps the most remarkable and characteristic parts of the
carboniferous holoptychius were its jaws and teeth. As we might
readily conjecture from the great size and strength of the scales and
cranial plates of this fish, its dentition was of a correspondingly
massive type. The under jaw, with the usual corrugated ornament,
frequently exceeded a foot in length, and displayed along its upper
edge a thick-set group of teeth. Of these there were two kinds one
of a smaller size and more blunted form, with short indented furrows
at their base; the other of a greatly more formidable size, grouped
at intervals among the smaller ones. The front end of each under jaw
bore one of these long conical tusks, serving as it were to guard the
entrance of the mouth. Each of the larger teeth had a base strongly
marked with longitudinal furrows, and sank deep into the jaw, with
the bone of which it sometimes anchylosed.[47] The part of the tooth
above this socket had an oval form, so flattened as to present two
cutting edges, one facing the front, the other the back of the mouth,
and meeting at the upper end of the tooth which was sharp and pointed.
Such large conical tusks may frequently be obtained, having a length
of two or three inches, while occasionally they range as high as six
or seven, the smaller teeth seldom reaching so much as an inch. It
is difficult to see how, with such a formidable dentition, the jaws
could readily close. In some specimens I have seen deep hollows beside
the bases of the teeth, which may possibly have received those of the
opposite jaw, but the gigantic tusks at the entrance of the mouth seem
to have stood high over the jaw, passing outside like those of the
wild-boar. If this be correct, the jaw of the holoptychius would unite
the mechanism of both the alligator and the crocodile--its recipient
hollows being analogous to the tooth-pits in the former tribe, and its
protruded teeth to the similarly exposed teeth of the latter.
[Footnote 47: I have seen detached teeth, wherein the length of
the root, or part imbedded in the jaw, tripled that of the exposed
part, sinking four or five inches into the bone without any trace of
anchylosis. Whether these huge tusks belonged to the upper or under
maxillary, I do not pretend to say, though no specimen of the under
jaw, which has ever come under my notice, would accommodate half of
such a deep-sunk base.]
[Illustration: Fig. 31.--Jaw of Holoptychius (_Rhizodus_.--Owen) from
Gilmerton, one-fourth nat. size; the large teeth along the middle part
of the jaw are here wanting.]
When we bring the microscope to bear upon the elucidation of the
structure of these ancient teeth, it seems as if our labour had but
just begun; and that so far from having by an external scrutiny
exhausted all that they have to show us, our knowledge of them can be
but scanty and superficial until we have studied them carefully under
a magnifying power. Microscopic sections of such organic remains are
prepared in the same way as those of the fossil woods already noticed;
and a more interesting or beautiful series of objects cannot be
conceived than a set of slices of these fossil-teeth.
Viewed, then, in longitudinal section from base to point, the part
above the fluted root of one of the large teeth of the holoptychius is
seen to consist of minute hair-like fibres of extreme tenuity, which
proceed in straight lines from the outer surface to the interior. At
right angles to these, and parallel with the outer edges, there is
a set of dark widely-placed lines conforming to the outline of the
tooth, like so many long sugar-loaf shaped caps, placed within each
other. When this part is cut across, and viewed in transverse section,
the tooth is observed to be of a flattened oval form, with the same
fine fibres or tubes radiating from the centre, and traversed by the
same dark bands which now assume the form of concentric rings. The
appearance thus presented reminds one at once of a cross section
of some dicotyledonous tree, the dark bands resembling the annual
layers of growth, and like these resulting from a similar thickening
of the internal tissue. The upper part of the tooth is solid and the
concentric rings few; the middle exhibits an increase of the rings, and
possesses, moreover, a hollow centre or pulp-cavity,[48] with the usual
diverging fibres. Here the oval form is well shown, and the encircling
rings are considerably flattened at the ends of the long axis.
[Footnote 48: This hollow centre may be seen occasionally filled up by
a sharp conical tooth like the _phragmocone_ of a belemnite.]
The lower portion of the tooth exhibits a much more complicated
texture. Externally it is marked by deep longitudinal furrows, that
run down the enamelled sides and sink into the jaw. When cut across
at this ribbed part, the tooth is found to present the most complex
and graceful internal structure. The prominent ridges between the
furrows are seen to be produced by crumpled folds of the substance of
the tooth, which roll inwards towards the centre, coalescing with each
other, and forming intricate groups of circling knots and folds. In
some places they seem all but separated from each other into little
circles, pierced with a central aperture, and recall the aspect of
the upper layer in the scale of megalichthys. Each of these loops and
folds presents a texture exactly similar to that of the upper part of
the tooth. The same minute hair-like tubes, darkened and thickened in
the long axis, radiate towards the centre; the same concentric bands
run from centre to circumference; so that the lower part of the tooth
seems, as it were, made up of a bundle of smaller teeth partially
melted into each other. Between these loops and folds circular meshes
frequently occur, and add to the complexity as well as the beauty of
the whole structure. One of these sections, with all its twisting
crumples, and folds, and knots, and coloured meshes, and encircled
rings, bears no small resemblance to an antique polished table that
has been cut out of the gnarled roots of a venerable oak. This complex
structure arose from the mode of growth of the tooth; each prominent
external ridge continually turning inwards down the furrow on either
side, and mingling in freakish knots with the folds that had gone
before.[49]
[Footnote 49: For an acquaintance with the remarkable teeth of this
ancient fish, more minute than it had been my good fortune to possess
before, I am indebted to a most interesting series of microscopical
preparations kindly lent me from his extensive collection by my friend
Mr. Alexander Bryson of Edinburgh.]
The internal bones of the holoptychius were of great size and strength,
as befitted such a bulky ganoid. Some of them had a singular style of
surface ornament, that somewhat resembled a frosted widow on a December
morning. Their internal structure was loose and cancellated; the endo-
being usually of a less compact texture than the exo-skeleton. Judging
from the size of such bones, the carboniferous holoptychius must have
been one of the bulkiest and most formidable denizens of the deep,
reaching sometimes to a length of twenty feet or even more. Such an
animal would have been, perhaps, quite a match for our hugest crocodile
or alligator, for it must have swum about with a litheness and agility
possessed by none of the saurian reptiles. Like that leviathan chosen
by the Almighty, in an age long subsequent, as an illustration of His
power and greatness, the holoptychius must have been king over all
the inhabitants of the sea, and the magnificent language of Job,
descriptive of the living animal, applies not less graphically to
the extinct one: "Who can open the doors of his face? his teeth are
terrible round about. His scales are his pride, shut up together as
with a close seal. One is so near to another, that no air can come
between them. He maketh the deep to boil like a pot: he maketh the sea
like a pot of ointment. He maketh a path to shine after him; one would
think the deep to be hoary."
Our survey has hitherto been directed to the denizens of carboniferous
lake, river, and sea, and we have found them to be alike important in
numbers and interesting in organization. It is otherwise, however,
when we turn in search of the denizens of the carboniferous lands.
The crowded trees and shrubs of the coal strata recalling as they do
old forest-covered swamps, might seem to indicate the probability of
a pretty numerous terrestrial fauna. Where are we to look for the
fossilized relics of land animals, if not in the remains of a submerged
land-surface? And yet, strange as it may seem, of the inhabitants of
the land during the Coal-measure period we know almost nothing. "We
have ransacked hundreds of soils replete with the fossil roots of
trees,--have dug out hundreds of erect trunks and stumps, which stood
in the position in which they grew,--have broken up myriads of cubic
feet of fuel, still retaining its vegetable structure,--and, after
all, we continue almost as much in the dark regarding the invertebrate
air-breathers of this epoch, as if the coal had been thrown down in
mid-ocean."[50]
[Footnote 50: Sir Charles Lyell's _Elements_, fifth edition, p. 406.]
The little land-shell already noticed as having been detected by Sir
Charles Lyell in the carboniferous rocks of Nova Scotia, seems to be
as yet the only air-breathing mollusc obtained from rocks of such
high antiquity. Insect remains have been detected in the English
coal-fields belonging to two or three species of beetles; while on the
Continent, wing-sheaths and other fragments of cockroaches, scorpions,
grasshoppers, locusts, crickets, &c., have been detected. But the most
remarkable traces of air-breathers consist in various indications of
the existence of reptiles during the Carboniferous era. Fragmentary
skeletons, with detached bones and plates, have been found in Bavaria
and America, together with long tracks of footprints, from which it
appears that during the time our coal-seams were forming, there swam
through the sluggish deltas, or crept amid the dank luxuriant foliage,
strange lizard-like forms, large enough to leave behind them on the
soft yielding mud or sand the impress of their double pair of toed
feet. But of these animals we have much to learn. Some of them have
bequeathed to us merely their dismembered broken bones; others have
left but the imprints of their toes. Yet even these remains, trifling
as they may seem, become of importance when we remember that they
demonstrate fishes not to have been the highest types of being during
the epoch of the Coal, and show that while the bulky holoptychius held
the supremacy of the waters, lizard-like forms of a less formidable
type seem, so far as we know, to have ruled it over the land.
In fine, then, no one can glance at a list of the carboniferous fauna
without perceiving either that the animated world of that ancient
epoch must have had a very different proportioning from what now
obtains, or that we have only a meagre and fragmentary record of it.
That the latter conclusion is the more philosophical will appear if we
reflect upon the many chances that exist against the entombment and
preservation of animal remains, especially of those peculiar to the
land. How very small a proportion of the remains of animals living in
our own country could be gathered from the surface-soil of any given
locality, and how very inadequate would be the meagre list of species
thus obtained, as representing the varied and extensive fauna of Great
Britain! In contrasting, then, the rich abundance of marine organisms
with the extreme paucity of terrestrial animals among the carboniferous
rocks, it would be too hasty to infer a corresponding disproportion
originally. It must be admitted that the rarity of air-breathers,
after such long-continued and extensive explorations among terrestrial
and lacustrine beds, presents a difficult problem, only (if at all)
to be cleared away by patient and persevering investigation. With
this preliminary caution, we may regard the carboniferous fauna as
peculiarly rich in marine species. The sea-bottoms swarmed with
stone-lilies, cup-corals, and net-like bryozoa, mingled with the
various tribes of molluscan life--the brachiopods with their long
ciliated arms; the bivalves and gastropods with their coloured shells
that recall some of the most familiar objects of our shores; and the
cephalopods with their groups of siphonated chambers, straight as in
the orthoceras, or gracefully coiled as in the goniatite. The seas
swarmed, too, with fishes belonging to the two great orders of ganoids
and placoids, the latter represented now by our sharks and rays,
though the exact type of the ancient genera is retained only by the
cestracion or Port-Jackson shark; the ganoids, with their strong armour
of bone, represented by but two genera, the lepidosteus of the American
rivers, and the polypterus of the Nile,--two fishes that seem but as
dwarfs when placed side by side with the gigantic holoptychius of the
coal-measures. The rivers and estuaries of the same period seem to have
been frequented by immense shoals of the smaller ganoidal fishes that
fed on decaying matter brought down from the land, and perhaps, too,
on the minute Crustacea that lay strewed by myriads along the bottom.
Into these busy scenes the bulkier monsters from the sea made frequent
migrations, perhaps in some cases ascending the rivers for leagues to
spawn, and returning again to their places at the mouth of the estuary
or in open sea. The rivers and lakes swarmed with small crustaceous
animals, and nourished, too, shells like those of our pearl-mussels.
The land--so luxuriantly clothed with vegetable forms--was hummed over
by beetles, chirupped over by grasshoppers and crickets, and crawled
over by four-footed reptiles, that united in their structure the lizard
and the frog. But of the general grade and proportions of its denizens
we still remain in ignorance. From all that yet appears, the scenery
of these forests must have been dark, silent, and gloomy, buried in a
solitude that was startled by no tiger's roar, no cattle's low, and
neither cheered with the melody of birds nor gladdened by the presence
of man.
We have lingered, perhaps, too long over the remains of these old
carboniferous animals. But the delay may be not without its use if, by
thus bringing before us some of the more marked points in the structure
of creatures that for ages peopled our planet, it broaden our view of
creation; and by lifting the curtain from off a dim, distant period of
our world's long history, it show, amid all diversities of arrangement,
and all varieties of form, still the same grand principles of design,
and the same modes of working as those which we can see and compare
among the living forms around us. It is something to be assured that
the race of man has been preceded by many other races, lower indeed
in the scale of being, but manifesting, throughout the long centuries
of their existence, ideas of mechanism and contrivance still familiar
to us, and serving in this way to link the human era with those that
have gone before, as parts of one grand scheme carried on by one great
Creator.