or frame on which the opercules rest, and furnishes the base of the
pectorals, was represented in the _Asterolepis_, as in the sturgeon, by a
ring of strong osseous plates, which, in one of the two species of which
trace is to be found among the rocks of Thurso, were curiously fretted
on their external surfaces, and in the other species comparatively
smooth. The largest, or coracoidian plate of the ring, as it occurs in
the more ornate species, (fig. 38,) might be readily enough mistaken,
when seen with only its surface exposed for the ichthyodorulite of
some large fish, allied, mayhap, to the _Gyracanthus formosus_ of the
Coal Measures; but when detached from the stone, the hollow form and
peculiar striæ of the inferior surface serve to establish its true
character as a dermal plate. The diagonal furrowings which traversed
it, as the twisted flutings traverse a Gothic column moulded after the
type of the Apprentice Pillar in Roslin chapel, seem to have underlaid
the edge of the opercule; at least I find a similar arrangement in the
shoulder-plates of a large species of _Diplopterus_, which are deeply
grooved and furrowed where the opercule rested, as if with the design of
keeping up a communication between the branchiæ and the external element,
even when the gill-cover was pressed closely down upon them. And,—as in
these shoulder-plates of the _Diplopterus_ the furrows yield their place
beyond the edge of the opercule to the punctulated enamel common to the
outer surface of all the creature’s external plates and scales,—we find
them yielding their place, in the shoulder-plates of the _Asterolepis_,
to the starred tubercles.
[Illustration: Fig. 38.
SHOULDER (_i. e._ CORACOID?) PLATE OF ASTEROLEPIS.
(One third nat. size, linear.)]
[Illustration: Fig. 39.
DERMAL BONES OF ASTEROLEPIS.
(One third nat. size, linear.)]
[Illustration: Fig. 40.
INTERNAL BONES OF ASTEROLEPIS.
(One half nat. size, linear.)]
A few detached bones, that bear on their outer surfaces the dermal
markings, must have belonged to that angular-shaped portion of the head
which intervened between the cranial buckler and the intermaxillary bone;
but the key for assigning to them their proper place is still to find;
and I suspect that no amount of skill on the part of the comparative
anatomist will ever qualify him to complete the work of restoration
without it. I have submitted to the reader the cranial bucklers of
_five_ several genera of the ganoids of the Old Red Sandstone; but no
amount of study bestowed on these would enable even the most skilful
ichthyologist to restore a _sixth_; nor is the lateral area of the head,
which was, I find, variously occupied in each genus, less difficult to
restore than the buckler which surmounted it. Two of the more entire of
these dermal bones I have figured (fig. 39, _a_ and _b_) in the hope
of assisting future inquirers, who, were they to pick up all the other
plates, might yet be unable, lacking the figured ones, to complete the
whole. The curiously-shaped plate _a_, represented in its various sides
by the figures 1, 2, 3, is of an acutely angular form in the transverse
section, (the external surface, 1, forming an angle which varies from
thirty to forty-five degrees with the base, 3;) and as it lay, it is
probable when in its original place, immediately under the edge of the
cranial buckler, it may have served to commence the line of deflection
from the flat top of the head to the steep descent of the sides, just
as what are technically termed the _spur_-stones in a gable-head serve
to commence the line of deflection from the vertical outline of the
wall to the inclined line of the roof, or as the spring-stones of an
arch serve to commence the curve. A few internal bones in my possession
are curious, but exceedingly puzzling. The bone _a_, fig. 40, which
resembles a rib, or branchiostegous ray, of one of the ordinary fishes,
formed apparently part of that osseous _style_ which in fishes such as
the haddock and cod we find attached to the suite of shoulder-bones, and
which, according to Cuvier, is the analogue of the coracoidian bone,
and, according to Professor Owen, the analogue of the clavicle. Fig.
_b_ is a mere fragment, broken at both ends, but exhibiting, in a state
of good keeping, lateral expansions, like those of an ancient halbert.
Fig. _c_, 41, which is also a fragment, though a more considerable
one, bears in its thicker and straighter edge a groove like that of an
ichthyodorulite, which, however, the bone itself in no degree resembles.
Fig. _d_ is a flat bone, of a type common in the skeleton of fishes, but
which, in mammals, we find exemplified in but the scapulars. It seems,
like these, to have furnished the base to which some suite of movable
bones was articulated,—in all likelihood that proportion of the carnal
bonelets of the pectoral fins which are attached in the osseous fishes
to its apparent homologue, the radius. Fig. _e_, a slim light bone,
which narrows and thickens in the centre, and flattens and broadens at
each end, was probably a scapula or shoulder-blade,—a bone which in
most fishes _splices_ on, as a sailor would say, by squamose jointings,
to the coracoidian bone at the one end, and the super-scapular bone at
the other. As indicated by its size, it must have belonged to a small
individual: it is, however, twice as long, and about six times as bulky,
as the scapula of a large cod.
[Illustration: Fig. 41.
INTERNAL BONES OF ASTEROLEPIS.
(One third nat. size, linear.)]
[Illustration: Fig. 42.
ISCHIUM OF ASTEROLEPIS.
(One half nat. size, linear.)]
Of the bone represented in fig. 42, I have determined, from a Cromarty
specimen, the place and use: it formed the interior base to which
one of the ventral fins was attached. In all fishes the bones of the
hinder extremities are inadequately represented: in none do we find the
pelvic arch complete; and to that nether portion of it which we do find
represented, and which Professor Owen regards as the homologue of the
_os ischium_ or hip-bone, the homologues of the metatarsal and toe-bones
are attached, to the exclusion of the bones of the thigh and leg. In the
Abdominales,—fishes such as the salmon and carp,—that have the ventrals
placed behind the abdomen, in the position analogous to that in which
the hinder legs of the reptiles and mammals occur, the ischiatic bones
generally exist as flat triangular plates, with their heads either
turned _inwards_ and downwards, as in the herring, or _outwards_ and
downwards, as in the pike; whereas in some of the cartilaginous fishes,
such as the Rays and Sharks, they exist as an undivided cartilaginous
band, stretched transversely from ventral to ventral. And such, with
but an upward direction, appears to have been their position in the
_Asterolepis_. They seem to have united at the narrow neck A, over the
middle of the lower portion of the abdomen; and to the notches of the
flat expansion B,—notches which exactly resemble those of the immensely
developed carpal bones of the Ray,—five metatarsal bones were attached,
from which the fin expanded. It is interesting to find the number in this
ancient representative of the vertebrata restricted to five,—a number
greatly exceeded in most of the existing fishes, but which is the true
normal number of the vertebrate sub-kingdom as shown in all the higher
examples such as man, the _quadrumana_, and in most of the _carnaria_.
The form of this bone somewhat resembles that of the analogous bone in
those fishes, such as the perch and gurnard, cod and haddock, which have
their ventrals suspended to the scapular belt; but its position in the
Cromarty specimen, and that of the ventrals in the various specimens of
the Cœlacanth family in which their place is still shown, forbids the
supposition that _it_ was so suspended,—a circumstance in keeping with
all the existing geological evidence on the subject, which agrees in
indicating, that of the low type of fishes that have, monster-like, their
_feet_ attached to their necks, the Old Red Sandstone does not afford a
trace. This inferior type, now by far the most prevalent in the ichthyic
division of the animal kingdom, does not seem to have been introduced
until near the close of the Secondary period, long after the fish had
been degraded from its primal place in the fore front of creation. In
one of my specimens a few fragments of the rays are preserved, (fig. 43,
_b_.) They are about the eighth part of an inch in diameter: depressed
in some cases in the center, as if, over the internal hollow formed by
the decay of the cartilaginous centre, the bony crust of which they are
composed had given way; and, like the rays of the thornback, they are
thickened at the joints, and at the processes by which they were attached
to the ischiatic base. It may be proper, I should here state, that of
some of the internal bones figured above I have no better evidence that
they belonged to the _Asterolepis_, than that they occur in the same
beds with the dermal plates which bear the characteristic star-like
markings,—that they are of very considerable size,—and that they formed
no part of the known fishes of the formation.
[Illustration: Fig. 43.
a. _Single joint of ray of Thornback._
b. _Single joint of ray of Asterolepis._]
[Illustration: Fig. 44.
COPROLITES OF ASTEROLEPIS.
(Nat. Size.[18])]
On exactly the same grounds I infer that certain large coprolites of
common occurrence in the Thurso flagstones, which contain the broken
scales of Dipterians, and exhibit a curiously twisted form, (fig. 44,)
also belonged to the _Asterolepis_; and from these, that the creature
was carnivorous in its habits,—an inference which the character of
its teeth fully corroborates; and farther, that, like the sharks and
rays, and some of the extinct Enaliosaurs, it possessed the spiral
disposition of intestine. Paley, in his chapter on the compensatory
contrivances palpable in the structure of various animals, refers to a
peculiar substitutory provision which occurs in a certain amphibious
animal described in the Memoirs of the French Academy. “The reader
will remember,” he says, “what we have already observed concerning the
_intestinal_ canal,—that its length, so many times exceeding that of the
body, promotes the extraction of the chyle from the aliment, by giving
room for the lacteal vessels to act upon it through a greater space.
This long intestine, whenever it occurs, is in other animals disposed in
the abdomen from side to side, in returning folds. But in the animal now
under our notice, the matter is managed otherwise. The same intention is
mechanically effectuated, but by a mechanism of a different kind. The
animal of which I speak is an amphibious quadruped, which our authors
call the Alopecias or sea-fox. The intestine is straight from one end to
the other but in this straight, and consequently short intestine, is a
winding, cork-screw, spiral passage, through which the food, not without
several circumvolutions, and, in fact, by a long route, is conducted to
its exit. Here the shortness of the gut is _compensated_ by the obliquity
of the perforation.” This structure of intestine, which all the true
Placoids possess, and at least the Sturiones among existing Ganoids,
seems to have been an exceedingly common one during both the Palæozoic
and Secondary periods. It has left its impress on all the better
preserved coprolites of the Coal Measures, so abundant in the shales of
Newhaven and Burdie House, and on those of the Lias and Chalk. It seems
to be equally a characteristic of well nigh all the bulkier coprolites
of the Lower Old Red Sandstone.[19] In these, however, it manifests
a peculiar trait, which I have failed to detect in any of the recent
fishes; nor have I yet seen it indicated, in at least the same degree, by
the Carboniferous or Secondary coprolitic remains. In the bowels which
moulded the coprolites of Lyme-Regis, of the Chalk, and of the Newhaven
and Granton beds, a single screw must have winded within the cylindrical
tube, as a turnpike stair winds within its hollow shaft; and such also is
the arrangement in the existing Sharks and Rays; whereas the bowels which
moulded the coprolites of the Lower Old Red Sandstone must have been
traversed by triple or quadruple screws laid closely together, as we find
the stalk of an old-fashioned wine-glass traversed by its thickly-set
spiral lines of thread-like china. And so, while on the surface of both
the Secondary and Carboniferous coprolites there is space between the
screw-like lines for numerous cross markings that correspond to the
thickly set veiny branches which traverse the sides of the recent placoid
bowel, the entire surface of the Lower Old Red coprolites is traversed
by the spiral markings. Is there nothing strange in the fact, that after
the lapse of mayhap millions of years,—nay, it is possible, millions
of ages,—we should be thus able to detect at once general resemblance
and special dissimilarity in even the most perishable parts of the most
ancient of the Ganoids?
I must advert, in passing, to a peculiarity exemplified in the state of
keeping of the bones of this ancient Ganoid, in at least the deposits
of Orkney and Caithness. The original animal matter has been converted
into a dark-colored bitumen, which in some places, where the remains
lie thick, pervades the crevices of the rocks, and has not unfrequently
been mistaken for coal. In its more solid state it can hardly be
distinguished, when used in sealing a letter,—a purpose which it serves
indifferently well,—from black wax of the ordinary quality; when more
fluid, it adheres scarce less strongly to the hands than the coal-tar of
our gas-works and dock-yards. Underneath a specimen of _Asterolepis_,
first pointed out to me in its bed among the Thurso rocks by Mr. Dick,
and which, at my request, he afterwards raised and sent me to Edinburgh,
packed up in a box, there lay a quantity of thick tar, which stuck as
fast to my fingers, on lifting out the pieces of rock, as if I had laid
hold of the planking of a newly tarred yawl. What had been once the
nerves, muscles, and blood of this ancient Ganoid still lay under its
bones, and reminded me of the appearance presented by the remains of a
poor suicide, whose solitary grave, dug in a sandy bank in the north
of Scotland, had been laid open by the encroachments of a river. The
skeleton, with pieces of the dress still wrapped round it, lay at length
along the section; and, for a full yard beneath, the white dry sand was
consolidated into a dark-colored pitchy mass, by the altered animal
matter which had escaped from it, percolating downwards, in the process
of decay.
In consequence of the curious chemical change which has thus taken place
in the animal juices of the _Asterolepis_, its remains often occur in
a state of beautiful preservation: the pervading bitumen, greatly more
conservative in its effects than the oils and gums of an old Egyptian
undertaker, has maintained, in their original integrity, every scale,
plate, and bone. They may have been much broken ere they were first
committed to the keeping of the rock, or in disentangling them from
its rigid embrace; but they have, we find, caught no harm when under
its care. Ere the skeleton of the Bruce, disinterred after the lapse of
five centuries, was recommitted to the tomb, such measures were taken
to secure its preservation, that, were it to be again disinterred, even
after as many more centuries had passed, it might be found retaining
unbroken its gigantic proportions. There was molten pitch poured over
the bones, in a state of sufficient fluidity to permeate all the pores,
and fill up the central hollows, and which, soon hardening around
them, formed a bituminous matrix, in which they may lie unchanged for
a thousand years. Now, exactly such was the process to which nature
resorted with these gigantic skeletons of the Old Red Sandstone. Like the
bones of the Bruce, they are bones steeped in pitch; and so thoroughly
is every pore and hollow still occupied, that, when cast into the fire,
they flame like torches. Though black as jet, they still retain, too,
in a considerable degree, the peculiar _qualities_ of the original
substance. The late Mr. George Sanderson of Edinburgh, one of the most
ingenious lapidaries in the kingdom, and a thoroughly intelligent man,
made several preparations for me, for microscopic examination, from
the teeth and bones; and though they were by far the oldest vertebrate
remains he had ever seen, they exhibited, he informed me, in the
working, more of the characteristics of recent teeth and bone than any
other fossils he had ever operated upon. Recent bone when in the course
of being reduced on the wheel to the degree of thinness necessary to
secure transparency, is apt, under the heat induced by the friction, to
acquire a springy elasticity, and to start up from the glass slip to
which it has been cemented; whereas bone in the fossil state usually
lies as passive, in such circumstances, as the stone which envelopes
it. Mr. Sanderson was, however, surprised to find that the bone of the
_Asterolepis_ still retained its elasticity, and was scarce less liable,
when heated, to start from the glass,—a peculiarity through which he at
first lost several preparations. I have seen a human bone that had for
ages been partially embedded in a mass of adipocere, partially enveloped
in the common mould of a churchyard, exhibit two very different styles
of keeping. In the adipocere it was as fresh and green as if it had
been divested of the integuments only a few weeks previous; whereas the
portion which projected into the mould had become brittle and porous, and
presented the ordinary appearance of an old churchyard bone. And what the
adipocere had done for the human bone in this case, seems to have been
done for the bones of the _Asterolepis_ by the animal bitumen.
[Illustration: Fig. 45.
HYOID PLATE OF THURSO ASTEROLEPIS.[20]
(One fifth the nat. size, linear.)]
The size of the _Asterolepis_ must, in the larger specimens, have been
very great. In all those ganoidal fishes of the Old Red Sandstone that
had the head covered with osseous plates, we find that the cranial
buckler bore a certain definite proportion,—various in the several genera
and species,—to the length of the body. The drawing-master still teaches
his pupils to regulate the proportions of the human figure by the seven
head-lengths which it contains; and perhaps shows them how an otherwise
meritorious draftsman,[21] much employed half an age ago in drawing for
the wood-engraver, used to render his figures squat and ungraceful by
making them a head too short. Now, those ancient Ganoids which possessed
a cranial buckler may, we find, be also measured by head-lengths. Thus,
in the _Coccosteus decipiens_, the length of the cranial buckler from
nape to snout equalled one fifth the entire length of the creature
from snout to tail. The entire length of the _Glyptolepis_ was equal to
about five one half times that of its cranial buckler. The _Pterichthys_
was formed in nearly the same proportions. The _Diplopterus_ was fully
seven times the length of its buckler: and the _Osteolepis_ from six and
a half to seven. In all the cranial bucklers of the _Asterolepis_ yet
found, the snout is wanting. The very fine specimen figured in page 99
(fig. 28) terminates abruptly at the little plate between the eyes, the
specimen figured in page 98 (fig. 27) terminates at the upper line of
the eye. The terminal portion which formed the snout is wanting in both,
and we thus lack the measure, or _module_, as the architect might say,
by which the proportions of the rest of the creature were regulated.
We can, however, very nearly approximate to it. A hyoid plate in my
collection (fig. 45) is, I find, so exactly proportioned in size to the
cranial buckler, (fig. 28,) that it might have belonged to the same
individual; and by fitting it in its proper place, and then making the
necessary allowance for the breadth of the nether jaw, which swept two
thirds around it, and was surmounted by the snout, we ascertain that
the buckler, when entire, must have been, as nearly as may be, a foot
in length. If the _Asterolepis_ was formed in the proportions of the
_Coccosteus_, the buckler (fig. 28) must have belonged to an individual
five feet in length; if in the proportions of the _Pterichthys_ or
_Glyptolepis_, to an individual five and a half feet in length; and
if in those of the _Diplopterus_ or _Osteolepis_, to an individual of
from six and a half to seven feet in length. Now I find that the hyoid
plate can be inscribed—such is its form—in a semicircle, of which the
nail-shaped ridge in the middle (if we strike off a minute portion of the
sharp point, usually wanting in detached specimens) forms very nearly
the radius, and of which the diameter equals the breadth of the cranial
buckler, along a line drawn across at a distance from the nape, equal
to two thirds of the distance between the nape and the eyes. Thus, the
largest diameter of a hyoid plate which belonged to a cranial buckler a
foot in length is, I find, equal to seven one quarter inches, while the
length of its nape somewhat exceeds three five eighth inches. The nail of
the Stromness specimen measures five and a half inches. It must have run
along a hyoid plate eleven inches in transverse breadth, and have been
associated with a cranial buckler eighteen one eighth inches in length;
and the _Asterolepis_ to which it belonged must have measured from snout
to tail, if formed, as it probably was, in the proportions of its brother
Cœlacanth the _Glyptolepis_, eight feet three inches; and if in those
of the _Diplopterus_, from nine feet nine to ten feet six inches. This
oldest of Scottish fish—this earliest-born of the Ganoids yet known—was
at least as bulky as a large porpoise.
It was small, however, compared with specimens of the _Asterolepis_
found elsewhere. The hyoid plate figured in page 110, (fig. 36,)—a
Thurso specimen which I owe to the kindness of Mr. Dick,—measures
nearly fourteen inches, and the cranial buckler of the same individual,
fifteen one fourth inches, in breadth. The latter, when entire, must
have measured twenty-three one half inches in length; and the fish to
which it belonged, if formed in the proportions of the _Glyptolepis_,
ten feet six inches; and if in those of the _Diplopterus_, from twelve
feet five to thirteen feet eight inches in length. Did the shield still
exist in its original state as a buckler of tough, enamel-crusted bone,
it might be converted into a Highland target, nearly broad enough to
cover the ample chest of a Rob Roy or Allan M’Aulay, and strong enough to
dash aside the keenest broadsword. Another hyoid plate found by Mr. Dick
measures sixteen one half inches in breadth; and a cast in the British
Museum, from one of the Russian specimens of Professor Asmus, (fig. 46,)
twenty-four inches. The individual to which this last plate belonged
must, if built in the shorter proportions, have measured eighteen, and if
in the longer, twenty-three feet in length. The two hyoid plates of the
specimen of _Holoptychius_ in the British Museum measure but four and a
half inches along that transverse line in which the Russian _Asterolepis_
measures two feet, and the largest Thurso specimen sixteen inches and a
half. The maxillary bone of a cod-fish two and a half feet from snout to
tail measures three inches in length. One of the Russian maxillary bones
in the possession of Professor Asmus measures in length twenty-eight
inches. And that space circumscribed by the sweep of the lower jaw
which it took, in the Russian specimen, a hyoid plate twenty-four
inches in breadth to fill, could be filled in the two-and-a-half-feet
cod by a plate whose breadth equalled but an inch and a half. Thus, in
the not unimportant circumstance of size, the most ancient Ganoids yet
known, instead of taking their places, agreeably to the demands of the
development hypothesis, among the sprats, sticklebacks, and minnows of
their class, took their place among its huge basking sharks, gigantic
sturgeons, and bulky sword-fishes. They were giants, not dwarfs.
[Illustration: Fig. 46.
HYOID PLATE OF RUSSIAN ASTEROLEPIS.
(One twelfth the natural size, linear.)]