and Cœlacanth genera—we find represented externally by massy plates,
bearing, like those of the upper jaw, their range of teeth. As shown
in a well-preserved specimen of the lower jaw of _Holoptychius_, in
my possession, they were boxes of bone enclosing a bulky nucleus of
cartilage, which, in approaching towards the condyloid process, where
great strength was necessary, was thickly traversed by osseous cancelli,
and passed at the joint into true bone. It is in the under jaws of the
earlier Ganoids that we first detect a true union of the external with
the internal skeleton,—of the bony plates and teeth, which were _mere
plates and teeth of the skin_, with the osseous, granular walls which
enclosed at least all the larger pieces of the cartilaginous framework
of the interior. The jaws of the Rays and Sharks, formed of cartilage,
and fenced round on their sides and edges by their thin coverings of
polygonal, bony points, are wholly internal and skin-covered; whereas
the teeth, which rest on the soft cuticular integument right over them,
are as purely dermal as the surrounding shagreen. Teeth and shagreen
may, we find, be alike stripped off with the skin. Now, in the earlier
ganoidal jaw, two sides of the osseous box which it composed,—its outer
and under sides,—were mere dermal plates, representative of the skin
of the placoids, or of their shagreen; while the other two,—its upper
and inner sides,—seem to have been developments of the interior osseous
walls which covered the endo-skeletal cartilage. Nor is it unworthy of
notice, that the reptile fishes of the period had their _ichthyic_ teeth
ranged along the edge of an exterior _dermal_ plate which covered the
outer side of the jaw; whereas their _reptile_ teeth were planted on
a plate, apparently of interior development, which covered its upper
edge. It is further worthy of remark, that while the teeth of the dermal
plate,—themselves also dermal,—seem as if they had grown out of it, and
formed part of it,—just as the teeth of the Placoids grew out of the
skin on which they rest,—the _reptile_ teeth within rested in shallow
pits,—the first faint indications of true sockets.
[Illustration: Fig. 14.
UNDER PART OF HEAD OF OSTEOLEPIS.[16]]
[Illustration: Fig. 15.
HEAD OF OSTEOLEPIS, SEEN IN PROFILE.]
That space included within the arch formed by the sweep of the under
jaws, which we find occupied in the osseous fishes by the hyoid bones and
the branchiostegous rays, was filled up externally, in the Dipterians
and Cœlacanths, and in at least two genera of Cephalaspians, by dermal
plates; in some genera, such as the _Diplopterus_, by three plates; in
others, such as the _Holoptychius_ and _Glyptolepis_, by two; and in
the _Asterolepis_, as we shall afterwards see, by but a single plate.
In the _Osteolepis_ these plates were increased to five in number, by
the little plates 14, 14, (fig. 14,) which, however, may have been also
present in the _Diplopterus_, though my specimens fail to show them. The
general arrangement was of much elegance,—an elegance, however, which,
in the accompanying restorations, the dislocation of the free plates,
drawn apart to indicate their detached character, somewhat tends to
obscure. But the position of the eyes must have imparted to the animal
a sinister reptile-like aspect. The profile, (fig. 15,) the result, not
of a chance-drawn outline, arbitrarily filled up, but produced by the
careful arrangement in their proper places of actually existing plates,
serves to show how perfectly the dermo-skeletal parts of the creature
were developed. Some of the animals with which we are best acquainted, if
represented by but their cuticular skeleton, would appear simply as sets
of hoofs and horns. Even the tortoise or pengolin would present about the
head and limbs their gaps and missing portions; but the dermo-skeleton
of the _Osteolepis_, composed of solid bone, and burnished with enamel,
exhibited the outline of the fish entire, and, with the exception of
the eye, the filling up of all its external parts. Presenting outside,
in its original state, no fragment of skin or membrane, and with even
its most flexible organs sheathed in enamelled bone, the _Osteolepis_
must have very much resembled a fish carved in ivory; and, though so
effectually covered, it would have appeared, from the circumstance, that
it wore almost all its bone outside, as naked as the human teeth.
[Illustration: Fig. 16.
CRANIAL BUCKLER OF DIPLOPTERUS.]
[Illustration: Fig. 17.
CRANIAL BUCKLER OF DIPLOPTERUS.]
The cranial buckler of the _Diplopterus_ (fig. 16) somewhat resembled
that of its fellow-dipterian the _Osteolepis_, but exhibited greater
elegance of outline. My first perfect specimen, which I owe to the
kindness of Mr. John Miller, of Thurso, an intelligent geologist of the
north, reminded me, as it glittered in jet-black enamel on its ground
of pale gray, of those Roman cuirasses which one sees in old prints,
impaled on stakes, as the central objects in warlike trophies formed
of spoils taken in battle. The rounded snout represented the chest and
shoulders, the middle portion the waist, and the expansion at the nape
the piece of dress attached, which, like the Highland kilt, fell adown
the thighs. The addition of a fragment of a sleeve, suspended a little
over the eye orbits, 2, 2, seemed all that was necessary in order to
render the resemblance complete. But as I disinterred the buried edges of
the specimen with a graver, the form, though it grew still more elegant,
became less that of the ancient coat of armor; the snout expanded into
a semicircle; the eye orbits gradually deepened; and the entire fossil
became not particularly like any thing but the thing it once was,—the
cranial buckler of the _Diplopterus_. The print (fig. 17) exhibits its
true form. It consists of two main divisions, occipital (A) and frontal,
(C, fig. 16;) and in each of these we find a pair of smaller divisions,
with what seem to be indications of yet further division, marked, not by
lines, but by dots; though I have hitherto failed to determine whether
the plates which these last indicate possess their independent centres of
ossification. Not unfrequently, however, has the comparative anatomist
to seek the analogues of two bones in one; nor is it at least _more_
difficult to trace in the faint divisions of the cranial buckler of the
_Diplopterus_, the homologues of the occipital, frontal, parietal,
mastoid, and nasal bones, than to recognize the representatives of the
carpals of the middle and ring finger in man, in the cannon bone of the
fore leg of the ox. I may mention in passing, that the little central
plate of the frontal division, (1, fig. 16,) which so nearly corresponds
with that of the _Osteolepis_, occurred, though with considerable
variations of form and homology, and some slight difference of position,
in all the Ganoids of the Old Red Sandstone whose craniums were covered
with an osseous buckler, and that its place was always either immediately
between the eyes or a very little over them. Its never-failing recurrence
shows that it must have had _some_ meaning, though it may be difficult
to say what. In the _Coccosteus_ it takes the form of the male dovetail,
which united the nasal plate or snout to the plate representative
of the superior frontal. Of the cartilaginous box which formed the
interior skull of either _Osteolepis_, or _Diplopterus_, or, with but
one exception, of the interior skulls of any of their contemporaries, no
trace, as I have said, has yet been detected. The solitary exception in
the case is, however, one of singular interest.
[Illustration: Fig. 18.
a. _Palatal dart-head._
b. _Group of palatal teeth._]
In a collection of miscellaneous fragments sent me by Mr. Dick from the
rocks of Thurso, I detected patches of palatal teeth ranged in nearly the
quadratures of circles, and which radiated outwards from the rectangular
angle or centre, (fig. 18, _b_.) And with the patches there occurred
plates exactly resembling the barbed head of a dart, (_a_,) with which
I had been previously acquainted, though I had failed to determine their
character or place. The excellent state of keeping of some of Mr. Dick’s
specimens now enabled me to trace the patches with the dart-head, and
several other plates, to a curious piece of palatal mechanism, ranged
along the base of a ganoid cranium, covered externally by a brightly
enamelled buckler, and to ascertain the order in which patches and plates
occurred. And then, though not without some labor, I succeeded in tracing
the buckler with which they were associated to the _Dipterus_,—a fish
which, though it has engaged the attention of both Cuvier and Agassiz,
has not yet been adequately restored. It is on an ill-preserved Orkney
specimen of the cranial buckler of this Ganoid that the ichthyologist has
founded his genus _Polyphractus_; while groupes of its palatal teeth from
the Old Red of Russia he refers to a supposed Placoid,—the _Ctenodus_.
But in the earlier stages of palæontological research, mistakes of this
character are wholly unavoidable. The palæontologist who did avoid them
would be either very unobservant, or at once very rash and very fortunate
in his guesses. If, ere an entire skeleton of the _Ichthyosaurus_
had turned up, there had been found in different localities, in the
Liasic formation, a beak like that of a porpoise, teeth like that of
a crocodile, a head and sternum like that of a lizard, paddles like
those of a cetacean, and vertebræ like those of a fish, it would have
been greatly more judicious, and more in accordance with the existing
analogies, to have erected, provisionally at least, places specifically,
or even generically separated, in which to range the separate pieces,
than to hold that they had all united in one anomalous genus; though such
was actually the fact. And Agassiz, in erecting three distinct genera
out of the fragments of a single genus, has in reality acted at once
more prudently and more intelligently than if he had avoided the error
by rashly uniting parts which in their separate state indicate no tie of
connection.
[Illustration: Fig. 19.
CRANIAL BUCKLER OF DIPTERUS.]
[Illustration: Fig. 20.
BASE OF CRANIUM OF DIPTERUS.]
The cranial buckler of the _Dipterus_ (fig. 19) was, like that of the
_Diplopterus_, of great beauty. In some of the finer specimens, we find
the enamel ornately tatooed, within the more strongly-marked divisions,
by delicately traced lines, waved and bent, as if upon the principle
of Hogarth; and though the lateral plates are numerous and small, and
defy the homologies, we may trace in those of the central line, from the
snout to the nape, what seem to be the representatives of the frontal,
parietal, and occipital bones,—the parietals ranging, as in the skull
of the carp and in that of most of the mammals, in their proper place
in the medial line. But the under surface of the cranium, armed, as on
the upper surface, with plates of bone, exhibited an arrangement still
more peculiar, (fig. 20.) Its rectangular patches of palatal teeth, its
curious dart-like bone, placed immediately behind these, and attached,
as the dart-head is attached to the handle, to a broad lozenge-shaped
plate, with two strong osseous processes projecting on either side, forms
such a _tout ensemble_ as is unique among fishes. Even here, however,
there may be traced at least a shade of homological resemblance to the
bones which form the base of the osseous skull. The single lozenge-shaped
plate, (A,) with its dart-head, occupies the place of the basi-occipital
bone; the posterior portion of the vomer seems represented by a strong
bony ridge, extending towards the snout; two separate bones, each bearing
one of the angular patches of teeth, corresponds to the sphenoid bone
and its alæ; and attached laterally to each of these there is the strong
projecting bone, on which the lower jaw appears to have hinged, and
which apparently represents the lower part of the temporal bone. Not less
singular was the form of the creature’s under jaw, (fig. 21.) I know no
other fish-jaw, whether of the recent or the extinct races, that might
be so readily mistaken for that of a quadruped. It exhibits not only the
condyloid, but also the coronoid processes; and, save that it broadens
on its upper edges, where in mammals the grinders are placed, so as to
furnish field enough for angular patches of teeth, which correspond with
the angular patches in the palate, it might be regarded, found detached,
as at least a reptilian, if not mammalian, bone. The disposition of
the palatal teeth of the _Dipterus_ will scarce fail to remind the
mechanist of the style of grooving resorted to in the formation of
mill-stones for the grinding of flour; nor is it wholly improbable that,
in correspondence with the rotatory motion of the stones to which the
grooving is specially adapted, jaws so hinged may have possessed some
such power of lateral motion as that exemplified by the human subject in
the use of the molar teeth.
[Illustration: Fig. 21.
UNDER JAW OF DIPTERUS.]
The protection afforded by the osseous covering of both the upper and
under surface of the cranium of this ichthyolite has resulted, in
several instances, in the preservation, though always in a greatly
compressed state, of the cranium itself, and the consequent exhibition
of two very important cranial cavities, the brain-pan proper, and the
passage through which the spinal cord passed into the brain. In the
sturgeon the brain occupies nearly the middle of the head; and there
is a considerable part of the occipital region traversed by the spine
in a curved channel, which, seen in profile, appears wide at the nape,
but considerably narrower where it enters the brain-pan, and altogether
very much resembling the interior of a miniature hunting-horn. And such
exactly was the arrangement of the greater cavities in the head of the
_Dipterus_. The portion of the cranium which was overlaid by what may be
regarded as the occipital plate was traversed by a cavity shaped like a
Lilliputian bugle-horn; while the hollow in which the brain was lodged
lay under the two parietal plates, and the little elliptical plate in
the centre. The accompanying print, (fig. 22,) though of but slight
show, may be regarded by the reader with some little interest, as a not
inadequate representation of the most ancient brain-pan on which human
eye has yet looked,—as, in short, the type of cell in which, myriads of
ages ago, in at least one genus, that mysterious substance was lodged, on
whose place and development so very much in the scheme of creation was
destined to depend. The specimen from which the figure is taken was laid
open laterally by chance exposure to the waves on the shores of Thurso,
another specimen, cut longitudinally by the saw of the lapidary, yields a
similar section, but greatly more compressed in the cavities; on which,
of course, as unsupported hollows, the compression to which the entire
cranium had been exposed chiefly acted. When the top and bottom of a
box are violently forced together, it is the empty space which the box
encloses that is annihilated in consequence of the violence.
[Illustration: Fig. 22.
LONGITUDINAL SECTION OF HEAD OF DIPTERUS]
It is deserving of notice, that the analogies of the cranial cavities
in this ancient Ganoid should point so directly on the cranial cavities
of that special Ganoid of the present time which unites a true skull of
cartilage to a dermal skull of osseous plates,—a circumstance strongly
corroborative of the general evidence, negative and positive, on which
I have concluded that the true skulls of the first Ganoids were also
cartilaginous. It is further worthy of observation, that in all the
sections of the cranium of _Dipterus_ which I have yet examined, the
internal line is continuous, as in the Placoids, from nape to snout,
and that the true skull presents no trace of those cerebral vertebræ of
which skulls are regarded by Oken and his disciples as developments.
Historically at least, the progress of the ichthyic head seems to have
been a progress from simple cartilaginous boxes to cartilaginous boxes
covered with osseous plates, that performed the functions whether active
or passive, of internal bones; and then from external plates to the
interior bones which the plates had previously represented, and whose
proper work they had done.
The principle which rendered it necessary that the divisions which exist
in the dermal skulls of the first Ganoids should so closely correspond
with the divisions which exist in the internal skulls of the osseous
fishes of a greatly later period, does not seem to lie far from the
surface. Of the solid parts of the ichthyic head, a certain set of
pieces afford protection to the brain and cerebral nerves, and to some
of the organs of the senses, such as those of seeing and hearing; while
another certain set of pieces constitute the framework through which an
important class of functions, manducatory and respiratory, are performed.
The protective bones of merely passive function are fixed, whereas the
bones of active function, such as the jaws, the osseous framework of the
opercules, and the hyoid bones, are to the necessary extent free, _i.
e._ capable of independent motion. Of course, the detached character
necessary to the free cerebral bones would be equally necessary in
cerebral plates united dermally to the pieces of the cartilaginous
framework, which performed in the ancient fish the functions of these
free bones. And hence jaw plates, opercular plates, and hyoid plates,
whose homological relation with recent jaws and opercular and hyoid
bones cannot be mistaken. They were operative in performing identical
mechanical functions, and had to exist, in consequence, in identical
mechanical conditions. And an equally simple, though somewhat different
principle, seems to have regulated the divisions of the fixed cranial
bucklers of the Old Red Ganoids, and to have determined their homologies
with the fixed cerebral bones of the osseous fishes.
These cranial bucklers, extending from nape to snout, protected the
exposed upper surface of the cartilaginous skull, and conformed to it in
shape, as a helmet conforms to the shape of the head, or a breast-plate
to the shape of the chest. And as the cartilaginous heads resembled in
general outline the osseous ones, the buckler which covered their upper
surface resembled in general outline the upper surface of the osseous
skull. It was in no case entirely a flat plate; but in every species
rounded over the snout and in most species at the sides; and so, in
order that its characteristic proportions might be preserved throughout
the various stages of growth in the head which it covered, it had to be
formed from several distinct centres of ossification, and to extend in
area around the edges of the plates originated from these. The workman
finds no difficulty in adding to the size of a piece of straight wall,
whether by heightening or lengthening it; but he cannot add to the size
of a dome or arch, without first taking it down, and then erecting it
anew on a larger scale. In the domes and arches of the animal kingdom,
the problem is solved by building them up of distinct pieces, few or
many, according to the demands of the figure which they compose, and
rendering these pieces capable of increase along their edges. It is on
this principle that the Cystidea, the Echinidæ, the Chelonian carapace
and plastron, and the skulls of the osseous Vertebrata, are constructed.
It is also the principle on which the cranial bucklers of the ancient
Ganoids were formed.[17] And from the general resemblance in figure of
these bucklers to the upper surface of the osseous skull, the separate
parts necessary for the building up of the one were anticipated, by many
ages, in the building up of the other; just as we find external arches
of stone which were erected two thousand years ago, constructed on the
same principle, and relatively of the same parts, as internal arches of
brick built in the present age. Doubtless, however, with this mechanical
necessity for correspondence of parts in the formation of corresponding
erections, there may have mingled that regard for typical resemblance
which seems so marked a characteristic of the _style_, if I may so
express myself, in which the Divine Architect gives expression to his
ideas. The external osseous buckler He divided after the general pattern
which was to be exemplified, in latter times, in the divisions of the
internal osseous skull; as if in illustration of that “ideal exemplar”
which dwelt in his mind from eternity, and on the palpable existence
of which sober science has based deductions identical in their scope
and bearing with some of the sublimest doctrines of the theologian.
“The recognition,” says Professor Owen, “of an ideal exemplar for the
vertebrated animals, proves that the knowledge of such a being as man
existed before man appeared; for the Divine mind which planned the
archetype also foreknew all its modifications. The archetypal idea was
manifested in the flesh, under divers such modifications, upon this
planet, long prior to the existence of those animal species that actually
exemplify it.”
But while we find place in that geological history in which every
character is an organism, for the “ideal exemplar” of Professor Owen,
we find _no_ place in it for the vertebræ-developed skull of Professor
Oken. The true genealogy of the head runs in an entirely different line.
The nerves of the cerebral senses did not, we find, originate cerebral
vertebræ, seeing that the heads of the first and second geologic periods
had their cerebral nerves, but _not_ their cerebral vertebræ; and that
what are regarded as cerebral-vertebræ appear for the first time, not in
the early fishes, but in the reptiles of the Coal formation. The line of
succession through the fish, indicated by the Continental assertor of
the development hypothesis, is a line cut off. All the existing evidence
conspires to show that the placoid heads of the Silurian system were,
like the placoid heads of the recent period, mere cartilaginous boxes;
and that in the succeeding system there existed ganoidal heads, that
to the internal cartilaginous box added external plates of bone, the
homologues, apparently,—so far at least as the merely cuticular could
be representative of the endo-skeletal,—of the opercular, maxillary,
frontal, and occipital bones in the osseous fishes of a long posterior
period,—fishes that were not ushered upon the scene until after the
appearance of the reptile in its highest forms and of even the marsupial
quadruped.
THE ASTEROLEPIS, ITS STRUCTURE, BULK, AND ASPECT.
With the reader, if he has accompanied me thus far, I shall now pass
on to the consideration of the remains of the _Asterolepis_. Our
preliminary acquaintance with the cerebral peculiarities of a few of
its less gigantic contemporaries will be found of use in enabling us to
determine regarding a class of somewhat resembling peculiarities which
characterized this hugest Ganoid of the Old Red Sandstone.
[Illustration: Fig. 24.
_Dermal tubercles of Asterolepis_
(Mag. two diameters.)]