Cœlacanths, and of all the Dipterians, was covered with osseous
plates,—its body with osseous scales; and, as I have already had occasion
to mention, it is from the star-like tubercles by which the cerebral
plates were fretted that M. Eichwald bestowed on the creature its
generic name. Agassiz has even erected species on certain varieties in
the pattern of the stars, as exhibited on detached fragments; but I am
far from being satisfied that we are to seek in their peculiarities of
style the characters by which the several species were distinguished.
The stellar form of the tubercle seems to have been its normal or most
perfect form as it was also, with certain modifications, that of the
tubercle of the _Coccosteus_ and _Pterichthys_; but its development as
a complete star was comparatively rare: in most cases the tubercles
existed without the rays,—frequently in the insulated pap-like shape,
but not rarely confluent, or of an elongated or bent form; and when to
these the characteristic rays were added, the stars produced were of a
rather eccentric order,—stars somewhat resembling the shadows of stars
seen in water. Individual specimens have already been found, on which, if
we recognize the form of the tubercle as a specific character, several
species might be erected. The accompanying wood-cut (fig. 24) represents,
from a Thurso specimen, what seems to be the true normal pattern of
these cerebral carvings. Seen in profile (_b_) the tubercles resemble
little hillocks, perforated at their base by single lines of thickly-set
caves; while seen from above, (_a_,) the narrow piers of bone by which
the caves are divided take the form of rays. The reader will scarce fail
to recognise in this print the coral _Monticularia_ of Lamarck, or to
detect, in at least the profile, the peculiarity which suggested the name.
[Illustration: Fig. 25.
SCALES OF ASTEROLEPIS.
(Nat. size.)
a. _Inner surface of scale._
b. _Exterior surface._]
[Illustration: Fig. 26.
PORTION OF CARVED SURFACE OF SCALE.
(Mag. four diameters.)]
The scales which covered the creature’s body (fig. 25) were, in
proportion to its size, considerably smaller and thinner than those
of the _Holoptychius_, which, however, they greatly resemble in their
general style of sculpture. Each, on the lower part of its exposed
field, was, we see, fretted by longitudinal anastomosing ridges, which,
in the upper part, break into detached angular tubercles, placed with
the apex downwards, and hollowed, leaf-like, in the centre; while that
covered portion which was overlaid by the scales immediately above we
find thickly pitted by microscopic hollows, that give to this part of the
field, viewed under a tolerably high magnifying power, a honeycombed
appearance. The central and lower parts of the interior surface of the
scale (_a_) are in most of the specimens irregularly roughened; while a
broad, smooth band, which runs along the top and sides, and seems to have
furnished the line of attachment to the creature’s body, is comparatively
smooth. The exterior carvings, though they demand the assistance of
the lens to see them aright, are of singular elegance and beauty; as
perhaps the accompanying wood-cut, (fig. 26,) which gives a magnified
view of a portion of the scale immediately above (_b_) from the middle
of the honeycombed field on the right side, to where the anastomosing
ridges bend gracefully in their descent, may in some degree serve to
show. I have seen a richly inlaid coat of mail, which was once worn by
the puissant Charles the Fifth; but its elaborate carvings, though they
belonged to the age of Benvenuto Cellini, were rude and unfinished,
compared with those which fretted the armor of the _Asterolepis_.
[Illustration: Fig. 27.
CRANIAL BUCKLER OF ASTEROLEPIS.
(One fifth nat. size, linear.)]
The creature’s cranial buckler, which was of great size and strength,
might well be mistaken for the carapace of some Chelonian fish of no
inconsiderable bulk. The cranial bucklers of the larger Dipterians
were ample enough to have covered the corresponding part in the skulls
of our middle-sized market-fish, such as the haddock and whiting; the
buckler of a _Coccosteus_ of the extreme size would have covered, if
a little altered in shape, the upper surface of the skull of a cod,
but the cranial buckler of _Asterolepis_, from which the accompanying
wood-cut was taken, (fig. 27,) would have considerably more than covered
the corresponding part in the skull of a large horse; and I have at
least one specimen in my collection which would have fully covered
the front skull of an elephant. In the smaller specimens, the buckler
somewhat resembles a laborer’s shovel divested of its handle, and sorely
rust-eaten along its lower or cutting edge. It consisted of plates,
connected at the edges by flat squamous sutures, or, as a joiner might
perhaps say, _glued_ together in _bevelled_ joints. And in consequence
of this arrangement, the same plates which seem broad on the exterior
surface appear comparatively narrow on the interior one, and _vice
versa_; the occipital plate, (_a_,) which, running from the nape along
the centre of the buckler, occupies so considerable a space on its outer
surface, exhibits inside a superficies reduced at least one half. Like
nine tenths of its contemporaries, the _Asterolepis_ exhibits the little
central plate between the eyes; but the eye orbits, unlike those of the
_Coccosteus_, and of all the Dipterian genera, which were half-scooped
out of the cranial buckler, half-encircled by detached plates, were
placed completely within the field of the buckler,—a circumstance in
which they resemble the eye orbits of the _Pterichthys_, and, among
existing fish, those of the sea-wolf. The characteristic is also a
distinctive one in Cuvier’s second family of the Acanthopterygii,—the
“fishes with hard cheeks.” A deep line immediately over the eyes, which,
however, indicated no suture, but seems to have been merely ornamental,
forms a sort of rudely tatooed eyebrow; the marginal lines parallel to
the lateral edges of the buckler were also mere tatooings; but all the
others indicated joints which, though more or less anchylosed, had a
real existence. So flat was the surface, that the edge of a ruler rests
upon it, in my several specimens, both lengthwise and across; but it was
traversed by two flat ridges, which, stretching from the corners of the
latero-posterior, _i. e._ parietal, plates, (_b_, _b_,) converged at the
little plate between the eyes, while along the centre of the depressed
angle which they formed, a third ridge, equally flat with the others, ran
towards the same point of convergence from the nape. The three ridges,
when strongly relieved by a slant light, resemble not inadequately an
impression, on a large scale, of the Queen’s broad arrow.
[Illustration: Fig. 28.
INNER SURFACE OF CRANIAL BUCKLER OF ASTEROLEPIS.
(One fifth nat. size, linear.)]
The inner surface of the cranial buckler of _Asterolepis_, (fig.
28,)—that which rested on the cartilaginous box which formed the
creature’s interior skull,—stands out in bolder relief from the stone
than its outer surface, and forms a more picturesque object. Like the
inner surfaces of the bucklers of _Coccosteus_ and _Pterichthys_, but
much more thickly than these, it was traversed by minute channelled
markings, somewhat resembling those striæ which may be detected in the
flatter bones of the ordinary fishes, and which seem in these to be
mere interstices between the osseous fibres. And in the plates, as in
the bones, they radiate from the centres of ossification, which are
comparatively dense and massy, towards the thinner overlapping edges.
These radiating lines are equally well marked in the cerebral bones of
the human fœtus. The three converging ridges on the outer surface we
find on the inner surface also,—the lateral ones a little bent in the
middle, but so directly opposite those outside, that the thickening of
the buckler which takes place along their line is at least as much a
consequence of their inner as of their outer elevation over the general
platform. A fourth bar ran transversely along the nape, and formed the
cross beam on which the others rested; for the three longitudinal ridges
may be properly regarded as three strong beams, which, extending from
the transverse beam at the nape to the front, where they converged like
the spokes of a wheel at the nave, gave to the cranial roof a degree of
support of which, from its great flatness, it may have stood in need. In
cranial bucklers in which the average thickness of the plates does not
exceed three _eighth_ parts of an inch, their thickness in the centre of
the ridges exceeds three _quarters_. The head of the largest crocodile
of the existing period is defended by an armature greatly less strong
than that worn by the _Asterolepis_ of the Lower Old Red Sandstone. Why
this ancient Ganoid should have been so ponderously helmed we can but
doubtfully guess; we only know, that when nature arms her soldiery, there
are assailants to be resisted and a state of war to be maintained. The
posterior central plate, the homologue apparently of the occipital bone,
was curiously carved into an ornate massive leaf, like one of the larger
leaves of a Corinthian capital, and terminated beneath, where the stem
should have been, in a strong osseous knob, fashioned like a pike head.
Two plates immediately over it, the homologues of the superior frontal
bone, with the little nasal plate which, perched atop in the middle,
lay between the creature’s eyes, resembled the head and breast in the
female figure, at least not less closely than those of the “lady in the
lobster;” the posterior frontal plates in which the outer and nether
half of the eye orbits were hollowed formed a pair of sweeping wings,
and thus in the centre of the buckler we are presented with the figure
of an angel, robed and winged, and of which the large sculptured leaf
forms the body, traced in a style in no degree more rude than we might
expect to see exemplified on the lichen-encrusted shield of some ancient
tombstone of that House of Avenel which bore as its arms the effigies
of the Spectre Lady. Children have a peculiar knack in detecting such
resemblances; and the discovery of the angel in the cranium of the
_Asterolepis_ I owe to one of mine.
[Illustration: Fig. 29.
PLATE OF CRANIAL BUCKLER OF ASTEROLEPIS.]
It is on this inner side of the cranial buckler, where there are no such
pseudo-joinings indicated as on the external surface, that the homologies
of the plates of which it is composed can be best traced. It might be
well, however, ere setting one’s self to the work of comparison, to
examine the skulls of a few of the osseous fishes of our coast, and to
mark how very considerably they differ from one another in their lines of
suture and their general form. The cerebral divisions of the conger-eel,
for instance, are very unlike those of the haddock or whiting; and the
sutures in the head of the gurnard are dissimilarly arranged from those
in the head of the perch. And after tracing the general type in the
more anomalous forms, and finding, with Cuvier, that in even these the
“skull consists of the same bones, though much subdivided, as the skulls
of the other vertebrata,” we will be the better qualified for grappling
with the not greater anomalies which occur in the cranial buckler of the
_Asterolepis_. The occipital plate, _A_, _a_, _a_, (fig. 29,) occupies
its ordinary place opposite the centre of the nape; the two parietals, B,
B, rest beside it in their usual ichthyic position of displacement; the
superior frontal we find existing, as in the young of many animals, in
two pieces, C, C; the nasal plate I, placed immediately in advance of it,
is flanked, as in the cod, by the anterior frontals, D, D; the posterior
frontals, F, F, which, when viewed as in the print, from beneath, seem of
considerable size, and describe laterally and posteriorly about one half
the eye orbits, have their area on the exterior surface greatly reduced
by the overriding squamose sutures of the plates to which they join; and
lastly, two of these overlying plates, E, E,—which, occurring in the
line of the lateral bar or beam, are of great strength and thickness,
and lie for two thirds of their length along the parietals, and for the
remaining third along the superior frontals,—represent the mastoid bones.
Such, so far as I have been yet able to read the cranial buckler of the
_Asterolepis_, seem to be the homologies of its component plates.
[Illustration: Fig. 30.
PORTION OF UNDER JAW OF ASTEROLEPIS, (OUTER SIDE.)
(One half nat. size.)]
[Illustration: Fig. 31.
PORTION OF UNDER JAW OF ASTEROLEPIS, (INNER SIDE.)
(One half nat. size.)]
There were no parts of the animal more remarkable than its jaws. The
under jaws,—for the nether maxillary consisted, in this fish, as in
the placoid fishes, and in the quadrupeds generally, of two pieces
joined in the middle,—were, like those of the _Holoptychius_, boxes
of bone, which enclosed central masses of cartilage. The outer and
under sides were thickly covered with the characteristic star-like
tubercles; and along the upper margin or lip there ran a thickly-set row
of small broadly-based teeth, planted as directly on the edge of the
exterior plate as iron spikes on the upper edge of a gate (fig. 30.)
Mr. Parkinson expresses some wonder, in his work on fossils, that, in
a fine ichthyolite in the British Museum, not only the _teeth_ should
have been preserved, but also the _lips_; but we now know enough of
the construction of the ancient Ganoids to cease wondering. The _lips_
were formed of as solid bone as the teeth themselves, and had as fair a
chance of being preserved entire; just as the metallic rim of a _cogged_
wheel has as fair a chance of being preserved as the metallic cogs that
project from it. Immediately behind the front row,—in which the teeth
present the ordinary ichthyic appearance,—there ran a thinly-set row of
huge _reptile_ teeth, based on an interior platform of bone, which formed
the top of the cartilage-enclosing box composing the jaw. These were at
once bent outwards and twisted laterally, somewhat like nails that have
been drawn out of wood by the claw of a carpenter’s hammer, and bent
awry with the wrench, (fig. 31.) They were furrowed longitudinally from
point to base by minute thickly-set striæ and were furnished laterally,
in most of the specimens though not in all, with two sharp cutting
edges. The reptile had as yet no existence in creation; but we see its
future coming symbolized in the dentition of this ancient Ganoid: it,
as it were, shows us the _crocodile_ lying entrenched behind the fish.
The interior structure of these reptile teeth is very remarkable. In the
longitudinal section we find numerous cancelli, ranged lengthwise along
the outer edges, but much crossed, net-like, within,—greatly more open
towards the base than at the point,—and giving place in the centre to a
hollow space, occasionally traversed by a few slim osseous partitions.
In the transverse section these cancelli are found to radiate from
the open centre towards the circumference, like the spokes of a wheel
from the nave; and each spoke seems as if, like Aaron’s rod, it had
become instinct with vegetative life, and had sprouted into branch and
blossom. Seen in a microscope of limited field, that takes in, as in
the accompanying print, (fig. 32,) not more than a fourth part of the
section, the appearance presented is that of a well-trained wall tree.
And hence the generic name _Dendrodus_, given by Professor Owen to teeth
found detached in the deposits of Moray, when the creatures to which they
had belonged were still unknown,—a name, however, which will, I suspect,
be found synonymous rather with that of a family than of a genus; for
so far as I have yet examined, I find that the dendrodic or tree-like
tooth, was in at least the Old Red Sandstone, a characteristic of all
the Cœlacanth family. I may mention, however, as a curious subject of
inquiry, that the Cœlacanths of the Coal Measures seem to have had their
reptile teeth formed of pure ivory,—a substance, which I have not yet
detected among the reptile-fish of the Old Red. Towards the base of the
reptile teeth of _Asterolepis_, the interstices between the branches
greatly widen, as in the branches of a tree in winter divested of its
foliage, (fig. 33, _c_;) the texture also opens towards the base in the
_fish_-teeth, outside, in which, however, the pattern in the transverse
section is greatly less complex and ornate than that which the reptile
teeth exhibits. When cut across near the point, they appear each as a
thick ring, (_b_,) traversed by lines that radiate towards the centre;
when cut across about half way down, they somewhat resemble, seen under
a high magnifying power those cast-iron wheels on which the engineer
mounts his railway carriages, (_a_.) In the longitudinal section their
line of junction with the jaw is marked by numerous openings, but by no
line of division, and they appear as thickly dotted by what were once
canaliculi, or life points, as any portion of the dermal bone on which
they rest.
[Illustration: Fig. 32.
PORTION OF TRANSVERSE SECTION OF REPTILE TOOTH OF ASTEROLEPIS
a. _Nat. size._
b. _Mag. twelve diameters._]
[Illustration: Fig. 33.
A. _Section of Jaw of Asterolepis._
c. _Reptile tooth as shown in section._
a, b, & c. _Row of ichthyic teeth in dermal plate of jaw._
B. _Magnified representatives of ichthyic teeth, a and b, in_ A.]
It seems truly wonderful, when one considers it, to what minute and
obscure ramifications that variety of pattern which nature so loves to
maintain is found to descend. It descends in the fishes, both recent
and extinct, to even the microscopic structure of their teeth; and
we find, in consequence, not less variety of figure in the sliced
fragments of the teeth of the ichthyolites of a single formation, than
in the carved blocks of an extensive calico print-yard. Each _species_
has its own distinct pattern, as if, in all the individuals of which
it consisted, the same block had been employed to stamp it; and each
_genus_ its own general _type_ of pattern, as if the same radical idea,
variously altered and modified, had been wrought upon in all. In the
_Dendrodic_ (Cœlacanth?) family, for instance, it is the radical type,
that from a central nave there should radiate, spoke-like, a number
of arborescent branches; but in the several genera and species of the
family, the branches belong, if I may so express myself, to different
shrubs, and present dissimilar outlines. It has appeared to me, that
at least a _presumption_ against the transmutation of species might be
based on those inherent peculiarities of structure which are thus found
to pervade the entire texture of the framework of animals. If we find
erections differing from one another merely in external form, we have
no difficulty in conceiving how, by additions and alterations, they
might be brought to exhibit a perfect uniformity of plan and aspect:
_transmutation_,—_development_,—_progression_,—(if one may use such
terms,)—seem possible in such circumstances. But if the buildings differ
from each other, not only in external form, but also in every brick and
beam, bolt and nail, no mere scheme of external alteration could ever
induce a real resemblance. Every brick would have to be taken down, and
every beam and bolt removed. The problem could not be wrought by the
remodelling of an old house: the only mode of solving it would be by the
erection of a new one.
[Illustration: Fig. 34.
MAXILLARY BONE?
(One fourth nat. size, linear.)]
Of the upper maxillary bones of the _Asterolepis_, I only know that
a considerable fragment of one of the pieces, recognized as such by
Agassiz, has been found in the neighborhood of Thurso by Mr. Dick,
unaccompanied, however, by any evidence respecting its place or function.
It exhibits none of the characteristic tubercles of the dermal bones,
and no appearance of teeth; but is simply a long bent bone, resembling
somewhat less than the half of an ancient bow of steel or horn,—such
a bow as that which Ulysses bended in the presence of the suitors.
By some of the Russian geologists this bone was at first regarded as
a portion of the arm or wing of some gigantic _Pterichthys_. In the
accompanying print (fig. 34) I have borrowed the general outline from
that of a specimen of Professor Asmus, of which a cast may be seen in
the British Museum; while the shaded portion represents the fragment
found by Mr. Dick. The intermaxillary bones, like the dermal plates of
the lower jaw, were studded by star-like tubercles, and bristled thickly
along their lower edges with the ichthyic teeth, flanked by teeth of
the reptilian character. The opercules of the animal consisted, as in
the sturgeon, of single plates (fig. 35) of great massiveness and size,
thickly tubercled outside, without trace of joint or suture, and marked
on their under surface by channelled lines, that radiate, as in the other
plates, from the centre of ossification. That space along the nape which
intervened between the opercules, was occupied, as in the _Dipterus_ and
_Diplopterus_, by three plates, which covered rather the anterior portion
of the body than the posterior portion of the head, and which, in the
restoration of _Osteolepis_, (fig. 13,) appear as the plates, 9, 9, 9. I
can say scarce any thing regarding the lateral plates which lay between
the intermaxillaries and the cranial buckler, and which exist in the
_Osteolepis_, fig. 13, as the plates 2, 4, 5, 6, and 7; nor do I know how
the snout terminated, save that in a very imperfect specimen it exhibits,
as in the _Diplopterus_ and _Osteolepis_, a rounded outline, and was set
with teeth.
[Illustration: Fig. 35.
INNER SURFACE OF OPERCULUM OF ASTEROLEPIS.
(One fifth nat. size, linear.)]
[Illustration: Fig. 36.
HYOID PLATE.
(One ninth nat. size, linear.)]
That space comprised within the arch of the lower jaws, in which the
hyoid bone and branchiostegous rays of the osseous fishes occur, was
filled by a single plate of great size and strength, and of singular
form, (fig. 36;) and to this plate, existing as a steep ridge running
along the centre of the interior surface, and thickening into a massy
knob at the anterior termination, that nail-shaped organism, which I have
described as one of the most characteristic bones of the _Asterolepis_,
belonged. In the _Osteolepis_, the space corresponding to that occupied
by this hyoid plate was filled, as shown in fig. 14, by five plates of
not inelegant form; and the divisions of the arch resembled those of
a small Gothic window, in which the single central mullion parts into
two branches atop. In the _Holoptychius_ and _Glyptolepis_ there were
but two plates; for the central mullion, _i. e._ line of division,
did not branch atop; and in the _Asterolepis_, where there was no
line of division, the strong nail-like bone occupied the place of the
central mullion. The hyoidal armature of the latter fish was strongest
in the line in which the others were weakest. Each of the five hyoid
plates of the _Osteolepis_, or of the two plates of the _Glyptolepis_
or _Holoptychius_, had its own centre of ossification; and in the
single plate of _Asterolepis_, the centre of ossification, as shown by
the radiations of the fibre, was the nail-head. This head, placed in
immediate contact with the strong boxes of bone which composed the under
jaw, just where their central joining occurred, seems to have lent them
a considerable degree of support, which at such a juncture may have been
not unnecessary. In some of the nail-heads, belonging, it is probable, to
a different species of _Asterolepis_ from that in which the nail figured
in page 7, and the plate in the opposite page, occurred,—for its general
form is different, (fig. 37,)—there appear well-marked ligamentary
impressions closely resembling that little spongy pit in the head of the
human thigh-bone to which what is termed the round ligament is attached.
The entire hyoid-plate, viewed on its outer side, resembles in form the
hyoid-bone,—or cartilage rather,—of the spotted dog-fish, (_Scyllium
stellare_;) but its area was at least a hundred times more extensive
than in the largest _Scyllium_, and, like all the dermal plates of the
_Asterolepis_, it was thickly fretted by the characteristic tubercles.
In the Ray, as in the Sharks, the piece of thin cartilage of which this
plate seems the homologue, is a flat, semi-transparent disk; and there
is no part of the animal in which the progress of those bony molecules
which encrust the internal framework may be more distinctly traced, as if
in the act of creeping over what they cover, in slim threads or shooting
points,—and much resembling new ice creeping in a frosty evening over the
surface of a pool.
[Illustration: Fig. 37.
NAIL-LIKE BONE OF HYOID PLATE.
(One half nat. size.)]