Russian deposits, very extensively developed, there occur in considerable
abundance certain animal organisms; but for many years neither their
position nor character could be satisfactorily determined. By some they
were placed too high in the scale of organized being; by others too low.
Kutorga, a writer not very familiarly known in this country, described
the remains as those of mammals;—the Russian rocks contained, he said,
bones of quadrupeds, and, in especial, the teeth of swine: whereas
Lamarck, a better known authority, though not invariably a safe one,—for
he had a trick of dreaming when wide awake, and of calling his dreams
philosophy,—assigned to them a place among the corals. They belonged,
he asserted, as shown by certain star-like markings with which they are
fretted, to the Polyparia. He even erected for their reception a new
genus of Astrea, which he designated, from the little rounded hillock
which rises in the middle of each star, the genus _Monticularia_. It was
left to a living naturalist, M. Eichwald, to fix their true position
zoologically among the class of fishes, and to Sir Roderick Murchison
to determine their position geologically as ichthyolites of the Old Red
Sandstone.
Sir Roderick, on his return from his great Russian campaigns, in which
he fared far otherwise than Napoleon, and accomplished more, submitted
to Agassiz a series of fragments of these gigantic Ganoids; and the
celebrated ichthyologist, who had been introduced little more than
a twelvemonth before to the _Pterichthys_ of Cromarty, was at first
inclined to regard them as the remains of a large cuirassed fish of
the Cephalaspian type, but generically new. Under this impression he
bestowed upon the yet unknown ichthyolite of which they had formed part,
the name _Chelonichthys_, from the resemblance borne by the broken
plates to those of the carapace and plastron of some of the Chelonians.
At this stage, however, the Russian Old Red yielded a set of greatly
finer remains than it had previously furnished; and of these casts were
transmitted by Professor Asmus, of the University of Dorpat, to the
British and London Geological Museums, and to Agassiz. “I knew not at
first what to do,” says the ichthyologist, “with bones of so singular
a conformation that I could refer them to no known type.” Detecting,
however, on their exterior surfaces the star-like markings which had
misled Lamarck, and which he had also detected on the lesser fragments
submitted to him by Sir Roderick, he succeeded in identifying both the
fragments and bones as remains of the same genus and on ascertaining
that M. Eichwald had bestowed upon it, from these characteristic
sculpturings, the generic name _Asterolepis_, or star-scale, he suffered
the name which he himself had originated to drop. Even this second name,
however, which the ichthyolite still continues to bear, is in some degree
founded in error. Its true scales, as I shall by and by show, were not
stelliferous, but fretted by a peculiar style of ornament, consisting
of waved anastomosing ridges, breaking atop into angular-shaped dots,
scooped out internally like the letter V; and were evidently intermediate
in their character between the scales which cover the _Glyptolepis_
and those of the _Holoptychius_. And the stellate markings which M.
Eichwald graphically describes as minute paps rising out of the middle
of star-like wreaths of little leaflets, were restricted to the dermal
plates of the head.
Agassiz ultimately succeeded in classing the bones which had at first so
puzzled him, into two divisions—interior and dermal; and the latter he
divided yet further, though not without first lodging a precautionary
protest, founded on the extreme obscurity of the subject, into cranial
and opercular. Of the interior bones he specified two,—a super-scapular
bone, (_supra-scapulaire_,)—that bone which in osseous fishes completes
the scapular arch or belt, by uniting the scapula to the cranium; and
a maxillary or upper jaw-bone. But his world-wide acquaintance with
existing fishes could lend him no assistance in determining the places
of the dermal bones: they formed the mere fragments of a broken puzzle,
of which the key was lost. Even in their detached and irreducible state,
however, he succeeded in basing upon them several shrewd deductions. He
inferred, in the first place, that the _Asterolepis_ was not, as had
been at first supposed, a cuirassed fish, which took its place among
the Cephalaspians, but a strongly helmed fish of that Cœlacanth family
to which the _Holoptychius_ and _Glyptolepis_ belong; in the second,
that, like several of its bulkier cogeners, it was in all probability
a broad, flat-headed animal; and, in the third, that as its remains
are found associated in the Russian beds with numerous detached teeth
of large size,—the boar tusks of Kutorga—which present internally that
peculiar microscopic character on which Professor Owen has erected his
Dendrodic or tree-toothed family of fishes,—it would in all likelihood
be found that both bones and teeth belonged to the same group. “It
appears more than probable,” he said, “that one day, by the discovery of
a head or an entire jaw, it will be shown that the genera _Dendrodus_
and _Asterolepis_ form but one.” As we proceed, the reader will see how
justly the ichthyologist assigned to the _Asterolepis_ its place among
the Cœlacanths, and how entirely his two other conjectures regarding it
have been confirmed. “I have had in general,” he concluded, “but small
and mutilated fragments of the creature’s bones submitted to me, and
of these, even the surface ornaments not well preserved; but I hope
the immense materials with which the Old Red Sandstone of Russia has
furnished the savans of that country will not be lost to science; and
that my labors on this interesting genus, incomplete as they are, will
excite more and more the attention of geologists, by showing them how
ignorant we are of all the essential facts concerning the history of the
first inhabitants of our globe.”
I know not what the savans of Russia have been doing for the last few
years; but mainly through the labors of an intelligent tradesman of
Thurso, Mr. Robert Dick,—one of those working men of Scotland of active
curiosity and well-developed intellect, that give character and standing
to the rest,—I am enabled to justify the classification and confirm the
conjectures of Agassiz. Mr. Dick, after acquainting himself, in the
leisure hours of a laborious profession, with the shells, insects, and
plants of the northern locality in which he resides, had set himself to
study its geology; and with this view he procured a copy of the little
treatise on the Old Red Sandstone to which I have already referred, and
which was at that time, as Agassiz’s Monograph of the Old Red fishes had
not yet appeared, the only work specially devoted to the palæontology
of the system, so largely developed in the neighborhood of Thurso. With
perhaps a single exception,—for the Thurso rocks do not yet seem to have
yielded a _Pterichthys_,—he succeeded in finding specimens, in a state
of better or worse keeping, of all the various ichthyolites which I had
described as peculiar to the Lower Old Red Sandstone. He found, however,
what I had _not_ described,—the remains of apparently a very gigantic
ichthyolite; and, communicating with me through the medium of a common
friend, he submitted to me, in the first instance, drawings of his new
set of fossils; and ultimately, as I could arrive at no satisfactory
conclusion from the drawings, he with great liberality made over to me
the fossils themselves. Agassiz’s Monograph was not yet published; nor
had I an opportunity of examining, until about a twelvemonth after,
the casts, in the British Museum, of the fossils of Professor Asmus.
Besides, all the little information, derived from various sources, which
I had acquired respecting the Russian _Chelonichthys_,—for such was its
name at the time,—referred it to the cuirassed type, and served but to
mislead. I was assured, for instance, that Professor Asmus regarded
his set of remains as portions of the plates and paddles of a gigantic
_Pterichthys_, of from twenty to thirty feet in length. And so, as I had
recognized in the Thurso fossils the peculiarities of the _Holoptychian_
(Cœlacanth) family, I at first failed to identify them with the remains
of the great Russian fish. All the larger bones sent me by Mr. Dick were,
I found, cerebral; and the scales associated with these indicated, not
a cuirass-protected, but a scale-covered body and exhibited, in their
sculptured and broadly imbricated surfaces, the well-marked Cœlacanth
style of disposition and ornament. But though I could _not_ recognize in
either bones or scales the remains of one ichthyolite more of the Old
Red Sandstone, “that could be regarded as manifesting as peculiar a type
among fishes as do the Ichthyosauri and Plesiosauri among reptiles,”[8]
I was engaged at the time in a course of inquiry regarding the cerebral
development of the earlier vertebrata, that made me deem them scarce less
interesting than if I could. Ere, however, I attempt communicating to the
reader the result of my researches, I must introduce him, in order that
he may be able to set out with me to the examination of the _Asterolepis_
from the same starting-point, to the Cœlacanth family,—indisputably one
of the oldest, and not the least interesting, of its order.
[Illustration: Fig. 2.
a. _Shagreen of the Thornback (Raja clavata.)_
b. _Shagreen of Sphagodus,—a placoid of the Upper Silurian._[9]]
So far as is yet known, all the fish of the earliest fossiliferous
system belonged to the placoid or “_broad plated_” order,—a great
division of fishes, represented in the existing seas by the Sharks and
Rays,—animals that to an internal skeleton of cartilage unite a dermal
covering of points, plates, or spines of enamelled bone, and have their
gills fixed. The dermal or cuticular bones of this order vary greatly in
form, according to the species or family: in some cases they even vary,
according to their place, on the same individual. Those button-like
tubercles, for instance, with an enamelled thorn, bent like a hook,
growing out of the centre of each, which run down the back and tail, and
stud the pectorals of the thorn-back, (_Raja clavata_,) differ very much
from the smaller thorns, with star-formed bases, which roughen the other
parts of the creature’s body; and the bony points which mottle the back
and sides of the sharks are, in most of the known species, considerably
more elongated and prickly than the points which cover their fins, belly,
and snout. The extreme forms, however, of the shagreen tubercle or plate
seem to be those of the upright prickle or spine on the one hand, and of
the slant-laid, rhomboidal, scale-shaped plate on the other. The minuter
thorns of the ray (fig. 2, _a_) exemplify the extreme of the prickly
type; the fins, abdomen, and anterior part of the head of the spotted
dog-fish (_Scyllium stellare_) are covered by lozenge-shaped little
plates, which glisten with enamel, and are so thickly set that they cover
the entire surface of the skin, (fig. 3, _b_,)—and these seem equally
illustrative of the scale-like form. They are shagreen points passing
into osseous scales, without, however, becoming really such; though they
approach them so nearly in the shape and disposition of their upper
disks, that the true scales, also osseous, of the _Acanthodes sulcatus_,
(fig. 3, _a_,) a Ganoid of the Coal Measures, can scarce be distinguished
from them, even when microscopically examined. It is only when seen in
section that the distinctive difference appears. The true scale of the
Acanthodes, though considerably elevated in the centre, seems to have
been planted on the skin; whereas the scale-like shagreen of the dog-fish
is elevated over it on an osseous pedicle or footstalk (fig. 5, _a_) as
a mushroom is elevated over the sward on its stem; and the base of the
stalk is found to resemble in its stellate character that of a shagreen
point of the prickly type. The apparent scale is, we find, a bony prickle
bent at right angles a little over its base, and flattened into a
rhomboidal disk atop.
[Illustration: Fig. 3.
a. _Scales of Acanthodes sulcatus._
b. _Shagreen of Scyllium stellare, (Snout.)_
(Mag. eight diameters.)]
[Illustration: Fig. 4.
a. _Scales of Cheiracanthus microlepidotus._
b. _Shagreen of Spinax Acanthias. (Snout.)_
(Mag. eight diameters.)]
[Illustration: Fig. 5.
a. _Section of shagreen of Scyllium stellare._
b. _Under surface of do._
c. _Section of scales of Cheiracanthus microlepidotus._
d. _Under surface of do._
(Mag. eight diameters.)]
In small fragments of shagreen, (fig. 2 _b_) which have been detected in
the bone-bed of the Upper Ludlow Rocks, (Upper Silurian,) and constitute
the most ancient portions of this substance known to the palæontologist,
the osseous tubercles are, as in the minuter spikes of the ray, of the
upright thorn-like type; they merely serve to show that the placoids
of the first period possessed, like those of the existing seas, an
ability of secreting solid bone on their cuticular surfaces; and that,
though at least such of them as have bequeathed to us specimens of
their dermal armature possessed it in the form farthest removed from
that of their immediate successors the ganoid fishes, they resembled
them not less in the substance of which their dermoskeletal, than in
that of which their endoskeletal, parts were composed. For the internal
skeleton in both orders, during these early ages, seems to have been
equally cartilaginous, and the cuticular skeleton equally osseous. In
the ichthyolitic formation immediately over the Silurians,—that of the
Lower Old Red Sandstone,—the Ganoids first appear; and the members of at
least one of the families of the deposit, the Acanths,—a family rich in
genera and species,—seem to have formed connecting links between this
second order and their placoid predecessors. They were covered with true
scales (fig. 4, _a_,) and their free gills were protected by gill-covers;
and so they must be regarded as real Ganoids but as the shagreen of the
spotted dog-fish nearly approaches, in form and character, to ganoid
scales, without being really such, the scales of this family, on the
other hand, approached equally near, without changing their nature, to
the shagreen of the Placoids, especially to that of the spiked dogfish,
(_Spinax Acanthias_.) (Fig. 4, _b_.) We even find on their under surfaces
what seems to be an approximation to the characteristic footstalk.
They so considerably thicken in the middle from their edges inwards,
(fig. 5, _c_,) as to terminate in their centres in obtuse points.
With these shagreen-like scales, the heads, bodies, and fins of all
the species of at least two of the Acanth genera,—_Cheiracanthus_ and
_Diplacanthus_,—were as thickly covered as the heads, bodies, and fins
of the sharks are with their shagreen; and so slight was the degree of
imbrication, that the portion of each scale overlaid by the two scales in
immediate advance of it did not exceed the one twelfth part of its entire
area. In the scale of the _Cheiracanthus_ we find the covered portion
indicated by a smooth, narrow band, that ran along its anterior edges,
and which the furrows that fretted the exposed surface did not traverse.
It may be added, that both genera had the anterior edge of their fins
armed with strong spines,—a characteristic of several of the Placoid
families.
[Illustration: Fig. 6.
a. _Scales of Osteolepis macrolepidotus._
b. _Scales of an undescribed species of Glyptolepis._[10]
(The single scales mag. two diameters;—the others nat. size.)]
In the Dipterian genera _Osteolepis_ and _Diplopterus_ the scales
were more unequivocally such than in the Acanths, and more removed
from shagreen. The under surface of each was traversed longitudinally
by a raised bar, which attached it to the skin, and which, in the
transverse section, serves to remind one of the shagreen footstalk.
They are, besides, of a rhomboidal form; and, when seen in the finer
specimens, lying in their proper places on what had been once the
creature’s body, they seem merely laid down side by side in line, like
those rows of glazed tiles that pave a cathedral floor; but on more
careful examination, we find that each little tile was deeply grooved
on its higher side and end, (for it lay diagonally in relation to the
head,) like the flags of a stone roof, (fig. 6, _a_,)—that its lateral
and anterior neighbors impinged upon it along these grooves to the
extent of about one third its area,—and that it impinged, in turn,
to the same extent on the scales that bordered on it posteriorly and
latero-posteriorly. Now, in the Cœlacanth family, (and on this special
point the foregoing remarks are intended to bear,) the scales, which were
generally of a round or irregularly oval form, (fig. 6, _b_,) overlapped
each other to as great an extent as in any of the existing fishes of
the Cycloid or Ctenoid orders,—to as great an extent, for instance, as
in the carp, salmon, or herring. In a slated roof there is no part on
which the slates do not lie double, and along the lower edge of each
tier they lie triple;—there is more of slate covered than of slate seen:
whereas in a tile-roof, the covered portion is restricted to a small
strip running along the top and one of the edges of each tile, and the
tiles do not lie double in more than the same degree in which the slates
lie triple. The scaly cover of the two genera of Dipterians to which
I have referred was a cover on the _tile_-roof principle; and this is
an exceedingly common characteristic of the scales of the Ganoids. The
scaly cover of the Cœlacanths, on the other hand, was a cover on the
_slate_-roof principle;—there was in some of their genera about one third
more of each scale covered than exposed; and this is so rare a ganoidal
mode of arrangement, that, with the exception of the _Dipterus_,—a genus
which, though it gives its name to the Dipterian sept, differed greatly
from every other Dipterian,—I know not, beyond the limits of the ancient
Cœlacanth family, a single Ganoid that possessed it. The bony covering of
the Cœlacanths was _farthest_ removed in character from shagreen, as that
of their contemporaries the Acanths approximated to it most nearly; they
were, in this respect, the two extremes of their order; and did we find
the Cœlacanths in but the later geological formations, while the Acanths
were restricted to the earlier, it might be argued by assertors of the
development hypothesis, that the amply imbricated, slate-like scale of
the latter had been developed in the lapse of ages from the shagreen
tubercle, by passing in its downward course—broadening and expanding
as it descended—through the minute, scarcely imbricated disks of the
Acanths, and the more amply imbricated tile-like rhombs of the Dipterians
and Palæonisci, until it had reached its full extent of imbrication in
the familiar modern type exemplified in both the Cœlacanths and the
ordinary fishes. But such is not the order which nature has observed;—the
two extremes of the ganoid scale appear together in the same early
formation: both become extinct at a period geologically remote; and the
ganoid scales of the existing state of things which most nearly resemble
those of ancient time are scales formed on the intermediate or tile-roof
principle.
The scales of the Cœlacanths were, in almost all the genera which
compose the family, of great size; in some species, of the greatest size
to which this kind of integument ever attained. Of a Cœlacanth of the
Coal Measures, the _Holoptychius Hibberti_, the scales in the larger
specimens were occasionally from five to six inches in diameter. Even
in the _Holoptychius nobilissimus_, in an individual scarcely exceeding
two and a half feet in length, they measured from an inch and a half to
an inch and three quarters each way. In the splendid specimen of this
last species, in the British Museum, there occur but fourteen scales
between the ventrals, though these lie low on the creature’s body, and
the head; and in a specimen of a smaller species,—the _Holoptychius
Andersoni_,—but about seventeen. The exposed portion of the scale was
in most species of the family curiously fretted by intermingled ridges
and furrows, pits and tubercles, which were either boldly relieved, as
in the _Holoptychius_, or existed, as in the _Glyptolepis_, as slim,
delicately chiselled threads, lines, and dots. The head was covered by
strong plates, which were roughened with tubercles either confluent or
detached, or hollowed, as in the _Bothriolepis_, into shallow pits. The
jaws were thickly set with an outer range of true fish teeth, and more
thinly with an inner range of what seem _reptile_ teeth, that stood up,
tall and bulky, behind the others, like officers on horseback seen over
the heads of their foot-soldiers in front. The _double_ fins,—pectorals
and ventrals,—were characterized each by a thick, angular, scale-covered
centre, fringed by the rays; and they must have borne externally
somewhat the form of the sweeping paddles of the Ichthyosaurian genus,—a
peculiarity shared also by the double fins of the _Dipterus_. The
_single_ fins, in all the members of the family of which specimens
have been found sufficiently entire to indicate the fact, were four in
number,—an anal, a caudal, and two dorsal fins; and, with the exception
of the anterior dorsal, which was comparatively small, and bent downwards
along the back, as if its rays had been distorted when young,[11] they
were all of large size. They crowded thickly on the posterior portion of
the body,—the anterior dorsal opposite the ventrals, and the posterior
dorsal opposite the anal fin. The fin-rays of the various members of the
family, and such of their spinous processes as have been detected, were
hollow tubular bones; or rather, like the larger pieces in the framework
of the Placoids, they were cartilaginous within, and covered externally
by a thin osseous crust or shell, which alone survives; and to this
peculiarity they owe their family name, Cœlacanth, or “hollow-spine.”
The internal hollow, _i. e._ cartilaginous centre, was, however,
equally a characteristic of the spinous processes of the _Coccosteus_.
In their general proportions, the Cœlacanths, if we perhaps except one
species,—the _Glyptolepis microlepidotus_,—were all squat, robust,
strongly-built fishes, of the Dirk Hatterick or Balfour-of-Burley type;
and not only in the larger specimens gigantic in their proportions, but
remarkable for the strength and weight of their armor, even when of but
moderate stature. The specimen of _Holoptychius nobilissimus_ in the
British Museum could have measured little more than three feet from snout
to tail when most entire; but it must have been nearly a foot in breadth,
and a bullet would have rebounded flattened from its scales. And such
was that ancient Cœlacanth family, of which the oldest of our Scotch
Ganoids,—the _Asterolepis_ of Stromness,—formed one of the members, and
which for untold ages has had no living representative.
Let us now enter on our proposed inquiry regarding the cerebral
development of the earlier vertebrata, and see whether we cannot
ascertain after what manner the first true brains were lodged, and
what those modifications were which their protecting box, the cranium,
received in the subsequent periods. Independently of its own special
interest, the inquiry will be found to have a direct bearing on our
general subject.
CEREBRAL DEVELOPMENT OF THE EARLIER VERTEBRATA. ITS APPARENT PRINCIPLE.