The Elfin-fish of Gawin Douglas.--The Fish of the Old Red Sandstone
scarcely less curious.--Place which they occupied indicated
in the present Creation by a mere Gap.--Fish divided into two
great Series, the Osseous and Cartilaginous.--Their distinctive
Peculiarities.--Geological Illustration of Dr. Johnson's shrewd
Objection to the Theory of Soame Jenyns.--Proofs of the intermediate
Character of the Ichthyolites of the Old Red Sandstone.--Appearances
which first led the Writer to deem it intermediate.--Confirmation
by Agassiz.--The _Osteolepis_.--Order to which this Ichthyolite
belonged.--Description.--_Dipterus._--_Diplopterus._--_Cheirolepis._
--_Glyptolepis._
Has the reader ever heard of the "griesly fisch" and the "laithlie
flood," described by that minstrel Bishop of Dunkeld "who gave rude
Scotland Virgil's page?" Both fish and flood are the extravagances
of a poet's dream. The flood came rolling through a wilderness of
bogs and quagmires, under banks "dark as rocks the whilk the sey
upcast." A skeleton forest stretched around, doddered and leafless;
and through the "unblomit" and "barrant" trees
"The quhissling wind blew mony bitter blast;"
the whitened branches "clashed and clattered;" the "vile water
rinnand o'erheid," and "routing as thonder," made "hideous trubil;"
and to augment the uproar, the "griesly fisch," like the fish of
eastern story, raised their heads amid the foam, and shrieked and
yelled as they passed. "The grim monsters fordeafit the heiring with
their sellouts;"--they were both fish and elves, and strangely noisy
in the latter capacity; and the longer the poet listened, the more
frightened he became. The description concludes, like a terrific
dream, with his wanderings through the labyrinths of the dead forest,
where all was dry and sapless above, and mud and marsh below,
and with his exclamations of grief and terror at finding himself
hopelessly lost in a scene of prodigies and evil spirits. And such
was one of the wilder fancies in which a youthful Scottish poet of
the days of Flodden indulged, ere taste had arisen to restrain and
regulate invention.
Shall I venture to say, that the ichthyolites of the Old Red
Sandstone have sometimes reminded me of the "fisch of the laithlie
flood?" They were hardly less curious. We find them surrounded, like
these, by a wilderness of dead vegetation, and of rocks upcast from
the sea; and there are the foot-prints of storm and tempest around
and under them. True, they must have been less noisy. Like the
"griesly fisch," however, they exhibit a strange union of opposite
natures. One of their families--that of the _Cephalaspis_--seems
almost to constitute a connecting link, says Agassiz, between fishes
and crustaceans. They had, also, their families of sauroid, or
reptile fishes--and their still more numerous families that unite the
cartilaginous fishes to the osseous. And to these last the explorer
of the Lower Old Red Sandstone finds himself mainly restricted. The
links of the system are all connecting links, separated by untold
ages from that which they connect; so that, in searching for their
representatives amid the existences of the present time, we find
but the gaps which they should have occupied. And it is essentially
necessary from this circumstance, in acquainting one's self with
their peculiarities, to examine, if I may so express myself, the
sides of these gaps,--the existing links at both ends to which the
broken links should have pieced,--in short, all those more striking
peculiarities of the existing disparted families which we find
united in the intermediate families that no longer exist. Without
some such preparation, the inquirer would inevitably share the fate
of the poetical dreamer of Dunkeld, by losing his way in a labyrinth.
In passing, therefore, with this object from the extinct to the
recent, I venture to solicit, for a few paragraphs, the attention of
the reader.
Fishes, the fourth great class in point of rank in the animal
kingdom, and, in extent of territory, decidedly the first, are
divided, as they exist in the present creation, into two distinct
series--the osseous and the cartilaginous. The osseous embraces
that vast assemblage which naturalists describe as "fishes properly
so called," and whose skeletons, like those of mammalia, birds,
and reptiles, are composed chiefly of a calcareous earth pervading
an organic base. Hence the durability of their remains. In the
cartilaginous series, on the contrary, the skeleton contains scarce
any of this earth: it is a framework of indurated animal matter,
elastic, semi-transparent, yielding easily to the knife, and, like
all mere animal substances, inevitably subject to decay. I have
seen the huge cartilaginous skeleton of a shark lost in a mass of
putrefaction in less than a fortnight. I have found the minutest
bones of the osseous ichthyolites of the Lias entire after the lapse
of unnumbered centuries.
The two series do not seem to precede or follow one another in
any such natural sequence as that in which the great classes of
the animal kingdom are arranged. The mammifer takes precedence of
the bird, the bird of the reptile, the reptile of the fish; there
is progression in the scale--the arrangement of the classes is
consecutive, not parallel. But in this great division there is
no such progression; the osseous fish takes no precedence of the
cartilaginous fish, or the cartilaginous, as a series, of the
osseous. The arrangement is parallel, not consecutive; but the
parallelism, if I may so express myself, seems to be that of a longer
with a shorter line;--the cartilaginous fishes, though much less
numerous in their orders and families than the other, stretch farther
along the scale in opposite directions, at once rising higher and
sinking lower than the osseous fishes. The cartilaginous order of the
sturgeons,--a roe-depositing tribe, devoid alike of affection for
their young, or of those attachments which give the wild beasts of
the forest partners in their dens,--may be regarded as fully abreast
of by much the greater part of the osseous fishes, in both their
instincts and their organization. The family of the sharks, on the
other hand, and some of the rays, rise higher, as if to connect the
class of fish with the class immediately above it--that of reptiles.
Many of them are viviparous, like the mammalia--attached, it is said,
to their young, and fully equal even to birds in the strength of
their connubial attachments. The male, in some instances, has been
known to pine away and die when deprived of his female companion.[O]
But then, on the other hand, the cartilaginous fishes, in some of
their tribes, sink as low beneath the osseous as they rise above them
in others. The suckers, for instance, a cartilaginous family, are
the most imperfect of all vertebral animals; some of them want even
the sense of sight; they seem mere worms, furnished with fins and
gills, and were so classed by Linnæus; but though now ascertained to
be in reality fishes, they must be regarded as the lowest link in the
scale--as connecting the class with the class _Vermes_, just as the
superior cartilaginous fishes may be regarded as connecting it with
the class _Reptilia_.
[Footnote O: Some of the osseous fishes are also viviparous--the
"viviparous blenny," for instance. The evidence from which the
supposed affection of the higher fishes for their offspring has been
inferred, is, I am afraid, of a somewhat equivocal character. The
love of the sow for her litter hovers, at times, between that of the
parent and that of the epicure; nor have we proof enough, in the
present state of ichthyological knowledge, to conclude to which side
the parental love of the fish inclines. The connubial affections of
some of the higher families seem better established. Of a pair of
gigantic rays (_Cephaloptera giorna_) taken in the Mediterranean,
and described by Risso, the female was captured by some fishermen;
and the male continued constantly about the boat, as if bewailing
the fate of his companion, and was then found floating dead.--See
Wilson's article Ichthyology, _Encyc. Brit._, seventh edition.]
Between the osseous and the cartilaginous fishes there exist some
very striking dissimilarities. The skull of the osseous fish is
divided into a greater number of distinct bones, and possesses more
movable parts, than the skulls of mammiferous animals: the skull of
the cartilaginous fish, on the contrary, consists of but a single
piece, without joint or suture. There is another marked distinction.
The bony fish, if it approaches in form to that general type which we
recognize amid all the varieties of the class as proper to fishes,
and to which, in all their families, nature is continually inclining,
will be found to have a tail branching out, as in the perch and
herring, from the bone in which the vertebral column terminates;
whereas the cartilaginous fish, if it also approach the general type,
will be found to have a tail formed, as in the sturgeon and dog-fish,
on both sides of the hinder portion of the spine, but developed much
more largely on the under than on the upper side. In some instances,
it is wanting on the upper side altogether. It may be as impossible
to assign reasons for such relations as for those which exist
between the digestive organs and the hoofs of the ruminant animals;
but it is of importance that they should be noted.[P] It may be
remarked, further, that the great bulk of fishes whose skeletons
consist of cartilage have yet an ability of secreting the calcareous
earth which composes bone, and that they are furnished with bony
coverings, either partial or entire. Their bones lie outside. The
thorn-back derives its name from the multitudinous hooks and spikes
of bone that bristle over its body; the head, back, and operculum of
the sturgeon are covered with bony plates; the thorns and prickles of
the shark are composed of the same material. The framework within is
a framework of mere animal matter; but it was no lack of the osseous
ingredient that led to the arrangement--an arrangement which we can
alone refer to the will of that all-potent Creator, who can transpose
his materials at pleasure, without interfering with the perfection
of his work. It is a curious enough circumstance, that some of the
osseous fishes, as if entirely to reverse the condition of the
cartilaginous ones, are partially covered with plates of cartilage.
They are bone within, and cartilage without, just as others are bone
without and cartilage within.
[Footnote P: Dr. Buckland, in his _Bridgewater Treatise_, assigns
satisfactory reasons for this construction of tail in sharks and
sturgeons. Of the fishes of these two orders, he states, "the former
perform the office of scavengers, to clear the water of impurities,
and have no teeth, but feed, by means of a soft, leather-like mouth,
capable of protrusion and contraction, on putrid vegetables and
animal substances at the bottom; and hence they have constantly to
keep their bodies in an inclined position. The sharks employ their
tail in another peculiar manner--to turn their body, in order to
bring their mouth, which is placed downwards beneath the head, into
contact with their prey. We find an important provision in every
animal, to give a position of ease and activity to the head during
the operation of feeding."--_Bridgewater Treatise_, p. 279, vol. i.,
first ed.]
But how apply all this to the Geology of the Old Red Sandstone?
Very directly. The ichthyolites of this ancient formation hold,
as has been said, an intermediate place, unoccupied among present
existences, between the two series, and in some respects resemble
the osseous, and in some the cartilaginous tribes. The fact reminds
one of Dr. Johnson's shrewd objection to the theory embraced by
Soame Jenyns in his _Free Inquiry_, and which was the theory also of
Pope and Bolingbroke. The metaphysician held, with the poet and his
friend, that there exists a vast and finely graduated chain of being
from Infinity to nonentity--from God to nothing; and that to strike
out a single link would be to mar the perfection of the whole.[Q]
The moralist demonstrated, on the contrary, that this chain, in
the very nature of things, must be incomplete at both ends--that
between that which does, and that which does not exist, there must
be an infinite difference--that the chain, therefore, cannot lay
hold on _nothing_. He showed, further, that between the greatest of
finite existences and the adorable Infinite there must exist another
illimitable void--that the boundless and the bounded are as widely
separated in their natures and qualities as the existent and the
non-existent--that the chain, in short, cannot lay hold on Deity. He
asserted, however, that not only is it thus incomplete at both ends,
but that we must regard it as well nigh as incomplete in many of its
intermediate links as at its terminal ones; that it is already a
broken chain, seeing that between its various classes of existence
myriads of intermediate existences might be introduced, by graduating
more minutely what must necessarily be capable of infinite gradation;
and that, to base an infidel theory on the supposed completeness of
what is demonstrably incomplete, and on the impossibility of a gap
existing in what is already filled with gaps, is just to base one
absurdity on another.[R] Now, we find the Geology of what may be
termed the second age of vertebrated existence (for the Lower Old Red
Sandstone was such) coming curiously in to confirm the reasonings
of Johnson. It shows us the greater part of the fish of an entire
creation thus insinuated between two of the links of our own.
[Footnote Q:
"See, through this air, this ocean, and this earth,
All matter quick, and bursting into birth;
Above, how high progressive life may go!
Around, how wide! how deep extend below!
Vast chain of being! which from God began--
Nature's ethereal, human angel, man,
Beast, bird, fish, insect--what no eye can see,
No glass can reach; from Infinite to thee--
From thee to nothing. On superior powers
Were we to press, inferior might on ours;
Or in the full creation leave a void,
Where, one step broken, the great scale's destroyed:
From Nature's chain, whatever link you strike,
Tenth, or ten thousandth, breaks the chain alike."
_Essay on Man._
]
[Footnote R: The following are the well-stated reasonings of Dr.
Johnson, a writer who never did injustice to an argument for want of
words to express it in:--
"The scale of existence from Infinity to nothing cannot possibly
have being. The highest being not infinite must be at an infinite
distance from Infinity. Cheyne, who, with the desire inherent
in mathematicians to reduce every thing to mathematical images,
considers all existence as a cone, allows that the basis is at an
infinite distance from the body, and in this distance between finite
and infinite there will be room forever for an infinite series of
indefinable existence.
"Between the lowest positive existence and nothing, whenever we
suppose positive existence to cease, is another chasm infinitely
deep, where there is room again for endless orders of subordinate
nature, continued forever and ever, and yet infinitely superior to
nonexistence.
"To these meditations humanity is unequal. But yet we may ask, not
of our Maker, but of each other, since on the one side creation,
whenever it stops, must stop infinitely below infinity, and on the
other infinitely above nothing, what necessity there is that it
should proceed so far either way--that being so high or so low should
ever have existed. We may ask, but I believe no created wisdom can
give an adequate answer.
"Nor is this all. In the scale, wherever it begins or ends, are
infinite vacuities. At whatever distance we suppose the next order of
beings to be above man, there is room for an intermediate order of
beings between them; and if for one order, then for infinite orders,
since every thing that admits of more or less, and consequently all
the parts of that which admits them, may be infinitely divided;
so that, as far as we can judge, there may be room in the vacuity
between any two steps of the scale, or between any two points of the
cone of being, for infinite exertion of infinite power."--_Review of
"A Free Inquiry."_]
It is now several years since I was first led to suspect that
the condition of the ichthyolites of the Old Red Sandstone was
intermediate. I have alluded to the comparative indestructibility
of the osseous skeleton, and the extreme liability to decay
characteristic of the cartilaginous one. Of a skeleton in part
osseous and in part cartilaginous, we must, of course, expect, when
it occurs in a fossil state, to find the indestructible portions
only. And when, in every instance, we find the fossil skeletons
of a formation complete in some of their parts, and incomplete in
others--the entire portions invariably agreeing, and the wanting
portions invariably agreeing also--it seems but natural to conclude
that an original difference must have obtained, and that the
existing parts, which we can at once recognize as bone, must have
been united to parts now wanting, which were composed of cartilage.
The naturalist never doubts that the shark's teeth, which he finds
detached on the shore, or buried in some ancient formation, were
united originally to cartilaginous jaws. Now, in breaking open all
the ichthyolites of the Lower Old Red Sandstone, with the exception
of those of the two families already described, we find that some
of the parts are invariably wanting, however excellent the state
of preservation maintained by the rest. I have seen every scale
preserved and in its place--one set of both the larger and smaller
bones occupying their original position--jaws thickly set with
teeth still undetached from the head--the massy bones of the skull
still unseparated--the larger shoulder-bone, on which the operculum
rests, lying in its proper bed--the operculum itself entire--and all
the external rays which support the fins, though frequently fine
as hairs, spreading out distinct as the fibres in the wing of the
dragon-fly, or the woody nerves in an oak-leaf. In no case, however,
have I succeeded in finding a single joint of the vertebral column,
or the trace of a single internal ray. No part of the internal
skeleton survives, nor does its disappearance seem to have had any
connection with the greater mass of putrescent matter which must
have surrounded it, seeing that the external rays of the fins show
quite as entire when turned over upon the body, as sometimes occurs,
as when spread out from it in profile. Besides, in the ichthyolites
of the chalk, no parts of the skeleton are better preserved than
the internal parts--the vertebral joints, and the internal rays.
The reader must have observed, in the cases of a museum of Natural
History, preparations of fish of two several kinds--preparations
of the skeleton, in which only the osseous parts are exhibited,
and preparations of the external form, in which the whole body is
shown in profile, with the fins spread to the full, and at least
half the bones of the head covered by the skin but in which the
vertebral column and internal rays are wanting. Now, in the fossils
of the chalk, with those of the other later formations, down to the
New Red Sandstone, we find that the skeleton style of preparation
obtains; whereas, in at least three fourths of the ichthyolites of
the Lower Old Red, we find only what we may term the external style.
I had marked, besides, another circumstance in the ichthyolites,
which seemed, like a nice point of circumstantial evidence, to give
testimony in the same line. The tails of all the ichthyolites, whose
vertebral columns and internal rays are wanting, are unequally lobed,
like those of the dog-fish and sturgeon, (both cartilaginous fishes,)
and the body runs on to nearly the termination of the surrounding
rays. The one-sided condition of tail exists, says Cuvier, in no
recent osseous fish known to naturalists, except in the bony pike--a
sauroid fish of the warmer rivers of America. With deference,
however, to so high an authority, it is questionable whether, the
tail of the bony pike should not rather be described as a tail set on
somewhat awry, than as a one-sided tail.
All these peculiarities I could but note as they turned up before me,
and express, in pointing them out to a few friends, a sort of vague,
because hopeless, desire, that good fortune might throw me in the
way of the one man of all the world best qualified to explain the
principle on which they occurred, and to decide whether fishes may be
at once bony and cartilaginous. But that meeting was a contingency
rather to be wished than hoped for--a circumstance within the
bounds of the possible, but beyond those of the probable. Could the
working-man of the north of Scotland have so much as dreamed that he
was yet to enjoy an opportunity of comparing his observations with
those of the naturalist of Neufchatel, and of having his inferences
tested and confirmed?
The opportunity did occur. The working-man did meet with Agassiz; and
many a query had he to put to him; and never, surely, was inquirer
more courteously entreated, or his doubts more satisfactorily
resolved. The reply to almost my first question solved the enigma of
nearly ten years' standing. And finely characteristic was that reply
of the frankness and candor of a great mind, that can afford to make
it no secret, that, in its onward advances on knowledge, it may know
to-day what it did not know yesterday, and that it is content to
"gain by degrees upon the darkness." "Had you asked me the question
a fortnight ago," said Agassiz, "I could not have replied to it.
Since then, however, I have examined an ichthyolite of the Old Red
Sandstone in which the vertebral joints are fortunately impressed
on the stone, though the joints themselves have disappeared, and
which, exactly resembling the vertebra? of the shark, must have
been cartilaginous." In a subsequent conversation, the writer
was gratified by finding most of his other facts and inferences
authenticated and confirmed by those of the naturalist. I shall
attempt introducing to the reader the peculiarities, general and
specific, of the ichthyolites to which these facts and observations
mainly referred, by describing such of the families as are most
abundant in the formation, and the points in which they either
resemble or differ from the existing fish of our seas.
[Illustration: _PLATE IV._]
Of these ancient families, the _Osteolepis_, or bony-scale,
(see Plate IV., fig. 1,) may be regarded as illustrative of the
general type. It was one of the first discovered of the Caithness
fishes, and received its name in the days of Cuvier, from the
osseous character of its scales, ere it was ascertained that it
had numerous contemporaries, and that to all and each of these the
same description applied. The scales of the fishes of the Lower
Old Red Sandstone, like the plates and detached prickles of the
purely cartilaginous fishes, were composed of a bony, not of a
horny, substance, and were all coated externally with enamel. The
circumstance is one of interest.
Agassiz, in his system of classification, has divided fishes into
four orders, according to the form of their scales; and his principle
of division, though apparently arbitrary and trivial, is yet found
to separate the class into great natural families, distinguished
from one another by other and very striking peculiarities. One kind
of scale, for instance, the placoid or broad-plated scale, is found
to characterize all the cartilaginous fishes of Cuvier except the
sturgeon;--it is the characteristic of an otherwise well-marked
series, whose families are furnished with skeletons composed of mere
animal matter, and whose gills open to the water by spiracles. The
fish of another order are covered by ctenoid or comb-shaped scales,
the posterior margin of each scale being toothed somewhat like the
edge of a saw or comb; and the order, thus distinguished, is found
wonderfully to agree with an order formed previously on another
principle of classification, the Acanthopterygii, or thorny-finned
order of Cuvier, excluding only the smooth-scaled families of
this previously formed division, and including, in addition to
it, the flat fish. A third order, the Cycloidean, is marked by
simple marginated scales, like those of the cod, haddock, whiting,
herring, salmon, &c.; and this order is found to embrace chiefly
the Malacopterygii, or soft-finned order of Cuvier--an order to
which all these well-known fish, with an immense multitude of others,
belong. Thus the results of the principle of classification adopted
by Agassiz wonderfully agree with the results of the less simple
principles adopted by Cuvier and the other masters in this department
of Natural History. Now, it is peculiar to yet a fourth order, the
Ganoidean, or shining-scaled order, that by much the greater number
of the genera which it comprises exist only in the fossil state. At
least five sixths of the whole were ascertained to be extinct several
years ago, at a time when the knowledge of fossil Ichthyology was
much more limited than at present: the proportions are now found to
be immensely greater on the side of the dead. And this order seems to
have included all the semi-osseous, semi-cartilaginous ichthyolites
of the Lower Old Red Sandstone: the enamelled scale is the
characteristic, according to Agassiz's principle of classification,
of the existences that filled the gap so often alluded to as existing
in the present creation. All their scales glitter with enamel: they
bore to this order the relation that the cartilaginous fish bear
to the Placoidean order, the thorny-finned fish to the Ctenoidean
order, and the soft-finned fish to the Cycloidean order. It also
included, with the semi-cartilaginous, the sauroid fish--those master
existences and tyrants of the earlier vertebrata; and both classes
find their representatives among the comparatively few ganoid fishes
of the present creation; the one in the sturgeon family, which of
all existing families approaches nearest in other respects to the
extinct semi-cartilaginous fishes; the other in the sauroid genus
_Lepidosteus_, to which the bony pike belongs. The head, back, and
sides of the sturgeon are defended, as has been already remarked, by
longitudinal rows of hard osseous bosses--the bony pike is armed
with enamelled osseous scales, of a stony hardness. It seems a
somewhat curious circumstance, that fishes so unlike each other in
their internal framework should thus resemble one another in their
bony coverings, and in some slight degree in their structure of
tail. One of the characteristics of sauroid fishes is the extreme
compactness and hardness of their skeleton.[S]
[Footnote S: "The sauroid or lizard-like fishes," says Dr. Buckland,
"combine in the structure, both of the bones and some of the soft
parts, characters which are common to the class of reptiles. The
bones of the skull are united by closer sutures than those of
common fishes. The vertebræ articulate with the spinous processes
of sutures, like the vertebræ of saurians; the ribs also articulate
with the extremities of the spinous process. The caudal vertebræ have
distinct chevron bones, and the general condition of the skeleton is
stronger and more solid than in other fishes: the air bladder also
is bifid and cellular, approaching to the character of lungs; and in
the throat there is a glottis, as in sirens and salamanders, and many
saurians."--Note to _Bridgewater Treatise_, p. 274, first edit.]
It requires skill such as that possessed by Agassiz, to determine
that the uncouth _Coccosteus_, or the equally uncouth _Pterichthys_,
of the Old Red Sandstone, with their long articulated tails and
tortoise-like plates, were _bona fide_ fishes; but there is no
possibility of mistaking the _Osteolepis_: it is obvious to the least
practised eye that it must have been a fish, and a handsome one. Even
a cursory examination, however, shows very striking peculiarities,
which are found, on further examination, to characterize not this
family alone, but at least one half the contemporary families
besides. We are accustomed to see vertebrated animals with the bone
uncovered in one part only,--that part the teeth,--and with the rest
of the skeleton wrapped up in flesh and skin. Among the reptiles,
we find a few exceptions; but a creature with a skull as naked as
its teeth,--the bone being merely covered, as in these, by a hard,
shining enamel,--and with toes also of bare enamelled bone, would
be deemed an anomaly in creation. And yet such was the condition
of the _Osteolepis_, and many of its contemporaries. The enamelled
teeth were placed in jaws which presented outside a surface as naked
and as finely enamelled as their own. (See Plate IV., fig. 5.) The
entire head was covered with enamelled osseous plates, furnished
inside like other bones, as shown by their cellular construction,
with their nourishing blood vessels, and perhaps their oil, and
which rested apparently on the cartilaginous box, which must have
enclosed the brain, and connected it with the vertebral column.
I cannot better illustrate the peculiar condition of the fins of
this ichthyolite than by the webbed foot of a water-fowl. The web
or membrane in all the aquatic birds with which we are acquainted
not only connects, but also covers the toes. The web or membrane in
the fins of existing fishes accomplishes a similar purpose; it both
connects and covers the supporting bones or rays. Imagine, however, a
webbed foot in which the toes--connected, but not covered--present,
as in skeletons, an upper and under surface of naked bone; and a very
correct idea may be formed, from such a foot, of the condition of fin
which obtained among at least one half the ichthyolites of the Lower
Old Red Sandstone. The supporting bones or rays seem to have been
connected laterally by the membrane; but on both sides they presented
bony and finely enamelled surfaces. (See Plate IV., fig. 6.) In this
singular class of fish, all was bone without, and all was cartilage
within; and the bone in every instance, whether in the form of jaws
or of plates, of scales or of rays, presented an external surface of
enamel.
The fins are quite a study. I have alluded to the connecting
membrane. In existing fish this membrane is the principal agent
in propelling the creature; it strikes against the water, as the
membrane of the bat's wing strikes against the air; and the internal
skeleton serves but to support and stiffen it for this purpose.
But in the fin of the _Osteolepis_, as in those of many of its
contemporaries, we find the condition reversed. The rays were so
numerous, and lay so thickly, side by side, like feathers in the
wing of a bird, that they presented to the water a surface of bone,
and the continuous membrane only served to support and bind them
together. In the fins of existing fish we find a sort of bat-wing
construction; in those of the _Osteolepis_ a sort of bird-wing
construction. The rays, to give flexibility to the organ which they
compose, were all jointed, as in the soft-finned fish--as in the
herring, salmon, and cod, for example; and we find in all the fins
the anterior ray rising from the body in the form of an angular
scale: it is a strong, bony scale in one of its joints, and a bony
ray in the rest. The characteristic is a curious one.
It is again necessary, in pursuing our description, to refer for
illustration to the purely cartilaginous fishes. In at least all
the higher orders of these, furnished with movable jaws, such as
the sturgeon, the ray, and the shark, the mouth is placed far below
the snout. The dog-fish and thorn-back are familiar instances.
Further, the mouth in bony fishes is movable on both the upper and
under side, like the beak of the parrot; in the higher cartilaginous
fishes it is movable, as in quadrupeds, on the under side only. In
all their orders, too, except in that of the sturgeon, the gills
open to the water by detached spiracles, or breathing-holes; but
in the sturgeon, as in the osseous fishes, there is a continuous
linear opening, shielded by an operculum, or gill-cover. In the
Osteolepis the mouth opened below the snout, but not so far below
it as in the purely cartilaginous fishes--not farther below it than
in many of the osseous ones--than in the genus Aspro, for instance,
or than in the genus Polynemus, or in even the haddock or cod. It
was thickly furnished with slender and sharply-pointed teeth. I have
hitherto been unable fully to determine whether, like the mouths of
the osseous fishes, it was movable on both sides; though, from the
perfect form of what seems to be the intermaxillary bone, I cannot
avoid thinking it was. The gills opened, as in the osseous fishes, in
continuous lines, and were covered by large bony opercules--that on
the enamelled side somewhat resemble round japanned shields.
But while the head of the _Osteolepis_, with its appendages, thus
resembled, in some points, the heads of the bony fishes, the tail,
like those of most of its contemporaries, differed in no respect
from the tails of cartilaginous ones, such as the sturgeon. The
vertebral column seems to have run on to well nigh the extremity
of the caudal fin, which we find developed chiefly on the under
side. The tail was a one-sided tail. Take into account with these
peculiarities--peculiarities such as the naked skull, jaws, and
operculum, the naked and thickly-set rays, and the unequally
lobed condition of tail--a body covered with scales, that glitter
like sheets of mica, and assume, according to their position, the
parallelogramical, rhomboidal, angular, or polygonal form--a lateral
line raised, not depressed--a raised bar on the inner or bony side
of the scales, which, like the doubled-up end of a tile, seems
to have served the purpose of fastening them in their places--a
general clustering of alternate fins towards the tail--and the
_tout ensemble_ must surely impart to the reader the idea of a very
singular little fish. The ventral fins front the space which occurs
between the two dorsals, and the anal fin the space which intervenes
between the posterior dorsal fin and the tail. The length of the
_Osteolepis_, in my larger specimens, somewhat exceeds a foot; in the
smaller, it falls short of six inches. There exist at least three
species of this ichthyolite, distinguished chiefly, in two of the
instances, by the smaller and larger size of their scales, compared
with the bulk of their bodies, and by punctulated markings on the
enamel in the case of the third. This last, however, is no specific
difference, but common to the entire genus, and to several other
genera besides. The names are, _Osteolepis macrolepidotus_, _O.
microlepidotus_, and _O. arenatus_.[T]
[Footnote T: To these there have since been added _Osteolepis major_,
_O. intermedius_, and _O. nanus_; the two latter, however, Agassiz
regards as doubtful.]
[Illustration: _PLATE V._]
Next to the _Osteolepis_ we may place the _Dipterus_, or double-wing,
of the Lower Old Red Sandstone, an ichthyolite first introduced to
the knowledge of geologists by Mr. Murchison, who, with his friend,
Mr. Sedgwick, figured and described it in a masterly paper on the
older sedimentary formations of the north of Scotland, which appeared
in the _Transactions of the Geological Society of London_ for 1828.
The name, derived from its two dorsals, would suit equally well,
like that of the Osteolepis, many of its more recently discovered
contemporaries. From the latter ichthyolite it differed chiefly in
the position of its fins, which were opposite, not alternate; the
double dorsals exactly fronting the anal and ventral fins. (See Plate
V., fig. 1.) The _Diplopterus_, a nearly resembling ichthyolite of
the same formation, also owes its name to the order and arrangement
of its fins, which, like those of the _Dipterus_, were placed
fronting each other, and in pairs. But the head, in proportion to
the body, was in greater size than in either the _Dipterus_ or
_Osteolepis_; and the mouth, as indicated by the creature's length
of jaw, must have been of much greater width. In their more striking
characteristics, however, the three genera seem to have nearly
agreed. In all alike, scales of bone glisten with enamel; their jaws,
enamel without and bone within, bristle thick with sharp-pointed
teeth; closely-jointed plates, burnished like ancient helmets, cover
their heads, and seem to have formed a kind of outer table to skulls
externally of bone and internally of cartilage; their gill-covers
consist each of a single piece, like the gill-cover of the sturgeon;
their tails were formed chiefly on the lower side of their bodies;
and the rays of their fins, enamelled like their plates and their
scales, stand up over the connecting membrane, like the steel or
brass in that peculiar armor of the middle ages, whose multitudinous
pieces of metal were fastened together on a groundwork of cloth or
of leather. All their scales, plates, and rays present a similar
style of ornament. The shining and polished enamel is mottled with
thickly-set punctures, or, rather, punctulated markings; so that a
scale or plate, when viewed through a microscope, reminds one of the
cover of a saddle. Some of the ganoid scales of Burdie House present
surfaces similarly punctulated.[U]
[Footnote U: There exists, according to Agassiz, only a single
species of _Dipterus--D. macrelepidotus_; whereas four species of
_Diplopterus_ have been enumerated--_D. affinis_, _D. borealis_, _D.
macrocephalus_, and _D. Agassizii_. The existence of the last named,
however, as a distinct species, is regarded as problematical by the
distinguished naturalist whose name has been affixed to it.]
The _Glyptolepis_, or carved scale, may be regarded as the
representative of a family of the Lower Old Red Sandstone, which,
differing very materially from the genera described, had yet many
traits in common with them, such as the bare, bony skull, the
bony scales, the naked rays, and the unequally sided condition of
tail. The fins, which were of considerable length in proportion
to their breadth of base, and present in some of the specimens
a pendulous-like appearance, cluster thick together towards the
creature's lower extremities, leaving the upper portion bare. There
are two dorsals placed as in the _Dipterus_ and _Diplopterus_--the
anterior directly opposite the ventral fin, the posterior directly
opposite the anal. The tail is long and spreading;--the rays, long
and numerously articulated, are comparatively stout at their base,
and slender as hairs where they terminate. The shoulder-bones
are of huge dimensions, the teeth extremely minute. But the most
characteristic parts of the creature are the scales. They are of
great size, compared with the size of the animal. An individual not
more than half a foot in length, the specimen figured, (see Plate
V., fig. 2,) exhibits scales fully three eighth parts of an inch
in diameter. In another more broken specimen there are scales a
full inch across, and yet the length of the ichthyolite to which
they belonged seems not to have much exceeded a foot and a half.
Each scale consists of a double plate, an inner and an outer.
The structure of the inner is not peculiar to the family or the
formation: it is formed of a number of minute concentric circles,
crossed by still minuter radiating lines--the one described, and
the other proceeding from a common centre. (See Plate V., fig. 5.)
All scales that receive their accessions of growth equally at their
edges exhibit, internally, a corresponding character. The outer plate
presents an appearance less common. It seems relieved into ridges
that drop adown it like sculptured threads, some of them entire,
some broken, some straight, some slightly waved, (see Plate V.,
fig. 3;) and hence the name of the ichthyolite. The plates of the
head were ornamented in a similar style, but their threads are so
broken as to present the appearance of dotted lines, the dots all
standing out in bold relief. My collection contains three varieties
of this family; one of them disinterred from out the Cromarty beds
about seven years ago, and the others only a little later, though
partly from the inadequacy of a written description, through which I
was led to confound the _Osteolepis_ with the _Diplopterus_, and to
regard the _Glyptolepis_ as the _Osteolepis_, I was not aware until
lately that the discovery was really such; and under the latter name
I described the creature in the Witness newspaper several weeks ere
it had received the name which it now bears. It was first introduced
to the notice of Agassiz, in Autumn last, by Lady Cumming of Altyre.
The species, however, was a different one from any yet found at
Cromarty.[V]
[Footnote V: There are three species of _Glyptolepis_--_G. elegans_,
_G. Leptopterus_, and _G. microlepidotus_.]
The _Cheirolepis_, or scaly pectoral, forms the representative of
yet another family of the Lower Old Red Sandstone, and one which
any eye, however unpractised, could at once distinguish from the
families just described. Professor Traill of the University of
Edinburgh, a gentleman whose researches in Natural History have
materially extended the boundaries of knowledge, and whose frankness
in communicating information is only equalled by his facility in
acquiring it, was the first discoverer of this family, one variety
of which, the _Cheirolepis Traillii_, bears his name. The figured
specimen (Plate VI., fig. 1) Agassiz has pronounced a new species,
the discovery of the writer. In all the remains of this curious
fish which I have hitherto seen, the union of the osseous with
the cartilaginous, in the general framework of the creature, is
strikingly apparent. The external skull, the great shoulder-bone,
and the rays of the fins, are all unequivocally osseous; the
occipital and shoulder-bones, in particular, seem of great strength
and massiveness, and are invariably preserved, however imperfect
the specimen in other respects; whereas, even in specimens the most
complete, and which exhibit every scale and every ray, however
minute, and show unchanged the entire outline of the animal, not a
fragment of the internal skeleton appears. The _Cheirolepis_ seems
to have varied from fourteen to four inches in length. When seen in
profile, the under line, as in the figured variety, seems thickly
covered with fins, and the upper line well nigh naked. The large
pectorals almost encroach on the ventral fins, and the ventrals on
the anal fin; whereas the back, for two thirds the entire length
of the creature, presents a bare rectilinear ridge, and the single
dorsal, which rises but a little way over the tail, immediately
opposite the posterior portion of the anal fin, is comparatively
of small size. The tail, which, in the general condition of being
developed chiefly on the lower side, resembles the tails of all
the creature's contemporaries, is elegantly lobed. The scales, in
proportion to the bulk of the body which they cover, are not more
than one twentieth the size of those of the _Osteolepis_. They are
richly enamelled, and range diagonally from the shoulder to the
belly in waving lines; and so fretted is each individual scale by
longitudinal grooves and ridges, that, on first bringing it under
the glass, it seems a little bunch of glittering thorns, though,
when more minutely examined, it is found to be present somewhat
the appearance of the outer side of the deep-sea cockle, with its
strongly marked ribs and channels, the point in which the posterior
point terminates representing the hinge. (See Plate VI., fig. 2.) The
bones of the head, enamelled like the scales, are carved into jagged
inequalities, somewhat resembling those on the skin of the shark,
but more irregular. The sculpturings seem intended evidently for
effect. To produce harmony of appearance between the scaly coat and
the enamelled occipital plates of bone, the surfaces of the latter
are relieved, where they border on the shoulders, into what seem
scales, just as the dead walls of a building are sometimes, for the
sake of uniformity, wrought into blind windows. The enamelled rays of
the fins are finished, if I may so speak, after the same style. They
lie thick upon one another as the fibres of a quill, and like these,
too, they are imbricated on the sides, so that the edge of each seems
jagged into a row of prickles. (See Plate VI., fig. 3.) The jaws of
the Cheirolepis were armed with thickly-set sharp teeth, like those
of its contemporaries, the _Osteolepis_ and _Diplopterus_.[W]
[Footnote W: There have been five species of _Cheirolepis_
enumerated--_C. Cummingiæ_, _C. splendens_, _C. Traillii_, _C.
unilateralis_ and _C. Uragus_. The _Cheirolepis splendens_ and _C.
unilateralis_ Agassiz regards as doubtful.]
[Illustration: _PLATE VI._]