THE DEVONIAN AND OLD RED SANDSTONE PERIOD.
Between the summit of the Ludlow formation and the strata which
are universally admitted to belong to the Carboniferous series
is a great system of deposits, to which the name of "Old Red
Sandstone" was originally applied, to distinguish them from certain
arenaceous strata which lie above the coal ("New Red Sandstone").
The Old Red Sandstone, properly so called, was originally described
and investigated as occurring in Scotland and in South Wales and
its borders; and similar strata occur in the south of Ireland.
Subsequently it was discovered that sediments of a different mineral
nature, and containing different organic remains, intervened
between the Silurian and the Carboniferous rocks on the continent
of Europe, and strata with similar palæontological characters to
these were found occupying a considerable area in Devonshire.
The name of "Devonian" was applied to these deposits; and this
title, by common usage, has come to be regarded as synonymous
with the name of "Old Red Sandstone." Lastly, a magnificent series
of deposits, containing marine fossils, and undoubtedly equivalent
to the true "Devonian" of Devonshire, Rhenish Prussia, Belgium,
and France, is found to intervene in North America between the
summit of the Silurian and the base of the Carboniferous rocks.
Much difficulty has been felt in correlating the true "Devonian
Rocks" with the typical "Old Red Sandstone"--this difficulty arising
from the fact that though both formations are fossiliferous, the
peculiar fossils of each have only been rarely and partially found
associated together. The characteristic crustaceans and many of the
characteristic fishes of the Old Red are wanting in the Devonian;
whilst the corals and marine shells of the latter do not occur in
the former. It is impossible here to enter into any discussion
as to the merits of the controversy to which this difficulty
has given origin. No one, however, can doubt the importance and
reality of the Devonian series as an independent system of rocks
to be intercalated in point of time between the Silurian and
the Carboniferous. The want of agreement, both lithologically
and palæontologically, between the Devonian and the Old Red,
can be explained by supposing that these two formations, though
wholly or in great part _contemporaneous_, and therefore strict
equivalents, represent deposits in two different geographical
areas, laid down under different conditions. On this view, the
typical Devonian rocks of Europe, Britain, and North America are
the deep-sea deposits of the Devonian period, or, at any rate, are
genuine marine sediments formed far from land. On the other hand,
the "Old Red Sandstone" of Britain and the corresponding "Gaspé
Group" of Eastern Canada represent the shallow-water shore-deposits
of the same period. In fact, the former of these last-mentioned
deposits contains no fossils which can be asserted positively
to be _marine_ (unless the Eurypterids be considered so); and
it is even conceivable that it represents the sediments of an
inland sea. Accepting this explanation in the meanwhile, we may
very briefly consider the general succession of the deposits of
this period in Scotland, in Devonshire, and in North America.
In Scotland the "Old Red" forms a great series of arenaceous and
conglomeratic strata, attaining a thickness of many thousands of
feet, and divisible into three groups. Of these, the _Lower Old
Red Sandstone_ reposes with perfect conformity upon the highest
beds of the Upper Silurian, the two formations being almost
inseparably united by an intermediate series of "passage-beds."
In mineral nature this group consists principally of massive
conglomerates, sandstones, shales, and concretionary limestones;
and its fossils consist chiefly of large crustaceans belonging to
the family of the _Eurypterids_, fishes, and plants. The _Middle
Old Red Sandstone_ consists of flagstones, bituminous shales,
and conglomerates, sometimes with irregular calcareous bands;
and its fossils are principally fishes and plants. It may be
wholly wanting, when the _Upper Old Red_ seems to repose
unconformably upon the lower division of the series. The _Upper
Old Red Sandstone_ consists of conglomerates and grits, along
with a great series of red and yellow sandstones--the fossils,
as before, being fishes and remains of plants. The Upper Old
Red graduates upwards conformably into the Carboniferous series.
The Devonian rocks of Devonshire are likewise divisible into a
lower, middle, and upper division. The _Lower Devonian_ or _Lynton
Group_ consists of red and purple sandstones, with marine fossils,
corresponding to the "Spirifer Sandstein" of Germany, and to the
arenaceous deposits (Schoharie and Cauda-Galli Grits) at the base
of the American Devonian. The _Middle Devonian_ or _Ilfracombe
Group_ consists of sandstones and flags, with calcareous slates
and crystalline limestones, containing many corals. It corresponds
with the great "Eifel Limestone" of the Continent, and, in a
general way, with the Corniferous Limestone and Hamilton group
of North America. The _Upper Devonian_ or _Pilton Group_, lastly,
consists of sandstones and calcareous shales which correspond with
the "Clymenia Limestone" and "Cypridina Shales" of the Continent,
and with the Chemung and Portage groups of North America. It
seems quite possible, also, that the so-called "Carboniferous
Slates" of Ireland correspond with this group, and that the former
would be more properly regarded as forming the summit of the
Devonian than the base of the Carboniferous.
In no country in the world, probably, is there a finer or more
complete exposition of the strata intervening between the Silurian
and Carboniferous deposits than in the United States. The following
are the main subdivisions of the Devonian rocks in the State of
New York, where the series may be regarded as being typically
developed (fig. 67):--
(1) _Cauda-Galli Grit_ and _Schoharie Grit_.--Considering the
"Oriskany Sandstone" as the summit of the Upper Silurian, the
base of the Devonian is constituted by the arenaceous deposits
known by the above names, which rest quite conformably upon the
Silurian, and which represent the Lower Devonian of Devonshire. The
_Cauda-Galli Grit_ is so called from the abundance of a peculiar
spiral fossil (_Spirophyton cauda-Galli_), which is of common
occurrence in the Carboniferous rocks of Britain, and is supposed
to be the remains of a sea-weed.
(2) The _Corniferous_ or _Upper Helderberg Limestone_.--A series
of limestones usually charged with considerable quantities of
siliceous matter in the shape of hornstone or chert (Lat. _cornu_,
horn). The thickness of this group rarely exceeds 300 feet; but
it is replete with fossils, more especially with the remains
of corals. The Corniferous Limestone is the equivalent of the
coral-bearing limestones of the Middle Devonian of Devonshire
and the great "Eifel Limestone" of Germany.
(3) The _Hamilton Group_--consisting of shales at the base
("Marcellus shales"); flags, shales, and impure limestones ("Hamilton
beds") in the middle; and again a series of shales ("Genesee
Slates") at the top. The thickness of this group varies from
200 to 1200 feet, and it is richly charged with marine fossils.
(4) The _Portage Group_.--A great series of shales, flags, and
shaly sandstones, with few fossils.
(5) The _Chemung Group_.--Another great series of sandstones and
shales, but with many fossils. The Portage and Chemung groups
may be regarded as corresponding with the Upper Devonian of
Devonshire. The Chemung beds are succeeded by a great series
of red sandstones and shales--the "Catskill Group"--which pass
conformably upwards into the Carboniferous, and which may perhaps
be regarded as the equivalent of the great sandstones of the
Upper Old Red in Scotland.
Throughout the entire series of Devonian deposits in North America
no unconformability or physical break of any kind has hitherto been
detected; nor is there any marked interruption to the current of
life, though each subdivision of the series has its own fossils.
No completely natural line can thus be indicated, dividing the
Devonian in this region from the Silurian on the one hand, and
the Carboniferous on the other hand. At the same time, there is
the most ample evidence, both stratigraphical and palæontological,
as to the complete independence of the American Devonian series
as a distinct life-system between the older Silurian and the
later Carboniferous. The subjoined section (fig. 76) shows
diagrammatically the general succession of the Devonian rocks
of North America.
[Illustration: Fig. 76. GENERALIZED SECTION OF THE DEVONIAN ROCKS
OF NORTH AMERICA.]
[Illustration: Fig. 77.--Restoration of _Psilophyton princeps_.
Devonian, Canada. (After Dawson.)]
As regards the _life_ of the Devonian period, we are now acquainted
with a large and abundant terrestrial _flora_--this being the
first time that we have met with a land vegetation capable of
reconstruction in any fulness. By the researches of Goeppert,
Unger, Dawson, Carruthers, and other botanists, a knowledge has
been acquired of a large number of Devonian plants, only a few
of which can be noticed here. As might have been anticipated,
the greater number of the vegetable remains of this period have
been obtained from such shallow-water deposits as the Old Red
Sandstone proper and the Gaspè series of North America, and few
traces of plant-life occur in the strictly marine sediments.
Apart from numerous remains, mostly of a problematical nature,
referred to the comprehensive group of the Sea-weeds, a large
number of Ferns have now been recognised, some being, of the
ordinary plant-like type (_Pecopteris, Neuropteris, Alethopteris,
Sphenopteris_, &c.), whilst others belong to the gigantic group
of the "Tree-ferns" (_Psaronius, Caulopteris_, &c.) Besides these
there is an abundant development of the singular extinct types of
the _Lepidodendroids_, the _Sigillarioids_, and the _Calamites_,
all of which attained their maximum in the Carboniferous. Of
these, the _Lepidodendra_ may be regarded as gigantic, tree-like
Club-mosses (_Lycopodiaceoe_); the _Calamites_ are equally gigantic
Horse-tails (_Equisetaceoe_); and the _Sigillarioids_, equally huge
in size, in some respects hold a position intermediate between
the Club-mosses and the Pines (Conifers). The Devonian rocks have
also yielded traces of many other plants (such as _Annularia,
Asterophyllites, Cardiocarpon_, &c.), which acquire a greater
pre-dominance in the Carboniferous period, and which will be
spoken of in discussing the structure of the plants of the
Coal-measures. Upon the whole, the one plant which may be considered
as specially characteristic of the Devonian (though not confined
to this series) is the _Psilophyton_ (fig. 77) of Dr Dawson.
These singular plants have slender branching stems, with sparse
needle-shaped leaves, the young stems being at first coiled up,
crosier-fashion, like the young fronds of ferns, whilst the old
branches carry numerous spore-cases. The stems and branches seem
to have attained a height of two or three feet; and they sprang
from prostrate "root-stocks" or creeping stems. Upon the whole,
Principal Dawson is disposed to regard _Psilophyton_ as a
"generalised type" of plants intermediate between the Ferns and
the Club-mosses. Lastly, the Devonian deposits have yielded the
remains of the first actual _trees_ with which we are as yet
acquainted. About the nature of some of these (_Ormoxylon_ and
_Dadoxylon_) no doubt can be entertained, since their trunks
not only show the concentric rings of growth characteristic of
exogenous trees in general, but their woody tissue exhibits under
the microscope the "discs" which are characteristic of the wood of
the Pines and Firs (see fig. 2). The singular genus _Prototaxites_,
however, which occurs in an older portion of the Devonian series
than the above, is not in an absolutely unchallenged position.
By Principal Dawson it is regarded as the trunk of an ancient
_Conifer_--the most ancient known; but Mr Carruthers regards it
as more probably the stem of a gigantic sea-weed. The trunks
of _Prototaxites_ (fig. 78, A) vary from one to three feet in
diameter, and exhibit concentric rings of growth; but its woody
fibres have not hitherto been clearly demonstrated to possess discs.
Before leaving the Devonian vegetation, it may be mentioned that
the hornstone or chert so abundant in the Corniferous limestone
of North America has been shown to contain the remains of various
microscopic plants (_Diatoms_ and _Desmids_). We find also in
the same siliceous material the singular spherical bodies, with
radiating spines, which occur so abundantly in the chalk flints,
and which are termed _Xanthidia_. These may be regarded as probably
the spore-cases of the minute plants known as _Desmidioe_.
[Illustration: Fig. 78.--A, Trunk of _Prototaxites Logani_, eighteen
inches in diameter, as seen in the cliff near L'Anse Brehaut,
Gaspé; B, Two wood-cells showing spiral fibres and obscure pores,
highly magnified. Lower Devonian, Canada. (After Dawson)]
The Devonian _Protozoans_ have still to be fully investigated.
True Sponges (such as _Astrtoeospongia, Sphoerospongia_, &c.)
are not unknown; but by far the commonest representatives of
this sub-kingdom in the Devonian strata are _Stromatopora_ and
its allies. These singular organisms (fig. 79) are not only very
abundant in some of the Devonian limestones--both in the Old World
and the New--but they often attain very large dimensions. However
much they may differ in minor details, the general structure of
these bodies is that of numerous, concentrically-arranged, thin,
calcareous laminæ, separated by narrow interspaces, which in turn
are crossed by numerous delicate vertical pillars, giving the whole
mass a cellular structure, and dividing it into innumerable minute
quadrangular compartments. Many of the Devonian _Stromatoporoe_
also exhibit on their surface the rounded openings of canals,
which can hardly have served any other purpose than that of
permitting the sea-water to gain ready access to every part of
the organism.
[Illustration: Fig. 79.--a, Part of the under surface of
_Stromatopora tuberculata_, showing the wrinkled basement membrane
and the openings of water-canals, of the natural size; b, Portion
of the upper surface of the same, enlarged; c, Vertical section of
a fragment, magnified to show the internal structure. Corniferous
Limestone, Canada. (Original.)]
[Illustration: Fig. 80.--_Cystiphyllum vesiculosum_, showing a
succession of cups produces by budding from the original coral.
Of the natural size. Devonian, America and Europe. (Original.)]
[Illustration: Fig. 81--_Zaphrentis cornicula_, of the natural
size. Devonian, America. (Original.)]
[Illustration: Fig. 82--_Heliophyllum exiguum_, viewed from in
front and behind. Of the natural size. Devonian, Canada. (Original.)]
[Illustration: Fig. 83.--Portion of a mass of _Crepidophyllum
Archiaci_, of the natural size. Hamilton Formation, Canada. (After
Billings.)]
No true _Graptolites_ have ever been detected in strata of Devonian
age; and the whole of this group has become extinguished--unless we
refer here the still surviving _Dictyonemoe_. The _Coelenterates_,
however, are represented by a vast number of _Corals_, of beautiful
forms and very varied types. The marbles of Devonshire, the Devonian
limestones of the Eifel and of France, and the calcareous strata
of the Corniferous and Hamilton groups of America, are often
replete with the skeletons of these organisms--so much so as to
sometimes entitle the rock to be considered as representing an
ancient coral-reef. In some instances the Corals have preserved
their primitive calcareous composition; and if they are embedded
in soft shales, they may weather out of the rock in almost all
their original perfection. In other cases, as in the marbles
of Devonshire, the matrix is so compact and crystalline that
the included corals can only be satisfactorily studied by means
of polished sections. In other cases, again, the corals have
been more or less completely converted into flint, as in the
Corniferous limestone of North America. When this is the case,
they often come, by the action of the weather, to stand out from
the enclosing rock in the boldest relief, exhibiting to the observer
the most minute details of their organization. As before, the
principal representatives of the Corals are still referable to
the groups of the _Rugosa_ and _Tabulata_. Amongst the Rugose
group we find a vast number of simple "cup-corals," generally
known by the quarrymen as "horns," from their shape. Of the many
forms of these, the species of _Cyathophyllum, Heliophyllum_
(fig. 82), _Zaphrentis_ (fig. 81), and _Cystiphyllum_ (fig. 80),
are perhaps those most abundantly represented--none of these
genera, however, except _Heliophyllum_, being peculiar to the
Devonian period. There are also numerous compound Rugose corals,
such as species of _Eridophyllum, Diphyphyllum, Syringopora,
Phillipsastroea_, and some of the forms of _Cyathophyllum_ and
_Crepidophyllum_ (fig. 83). Some of these compound corals attain
a very large size, and form of themselves regular beds, which
have an analogy, at any rate, with existing coral-reefs, though
there are grounds for believing that these ancient types differed
from the modern reef-builders in being inhabitants of deep water.
The "Tabulate Corals" are hardly less abundant in the Devonian
rocks than the _Rugosa_; and being invariably compound, they
hardly yield to the latter in the dimensions of the aggregations
which they sometimes form.
[Illustration: Fig. 84.--Portion of a mass of _Favosites
Gothlandica_, of the natural size. Upper Silurian and Devonian
of Europe and America. (Original.) Billings.]
[Illustration: Fig. 85.--Fragment of _Favosites hemispherica_,
of the natural size. Upper Silurian and Devonian of America.
(After Billings.)]
The commonest, and at the same time the largest, of these are
the "honeycomb corals," forming the genus _Favosites_ (figs.
84, 85), which derive both their vernacular and their technical
names from their great likeness to masses of petrified honeycomb.
The most abundant species are _Favosites Gothlandica_ and _F.
Hemispherica_, both here figured, which form masses sometimes
not less than two or three feet in diameter. Whilst _Favosites_
has acquired a popular name by its honey-combed appearance, the
resemblance of _Michelinia_ to a fossilised wasp's nest with the
comb exposed is hardly less striking, and has earned for it a
similar recognition from the non-scientific public. In addition
to these, there are numerous branching or plant-like Tabulate
Corals, often of the most graceful form, which are distinctive
of the Devonian in all parts of the world.
The _Echinoderms_ of the Devonian period call for little special
notice. Many of the Devonian limestones are "crinoidal;" and
the _Crinoids_ are the most abundant and widely-distributed
representatives of their class in the deposits of this period.
The _Cystideans_, with doubtful exceptions, have not been recognised
in the Devonian; and their place is taken by the allied group of
the "Pentremites," which will be further spoken of as occurring
in the Carboniferous rocks. On the other hand, the Star-fishes,
Brittle-stars, and Sea-urchins are all continued by types more
or less closely allied to those of the preceding Upper Silurian.
Of the remains of Ringed-worms (_Annelides_), the most numerous
and the most interesting are the calcareous envelopes of some
small tube-inhabiting species. No one who has visited the seaside
can have failed to notice the little spiral tubes of the existing
_Spirorbis_ growing attached to shells, or covering the fronds
of the commoner Sea weeds (especially _Fucus serratus_). These
tubes are inhabited by a small Annelide, and structures of a
similar character occur not uncommonly from the Upper Silurian
upwards. In the Devonian rocks, _Spirorbis_ is an extremely common
fossil, growing in hundreds attached to the outer surface of
corals and shells, and appearing in many specific forms (figs.
86 and 87); but almost all the known examples are of small size,
and are liable to escape a cursory examination.
[Illustration: Fig. 87.--a, _Spirobois omphalodes_, natural size
and enlarged. Devonian, Europe and America; b, _Spirorbis
Arkonensis_, of the natural size and enlarged; c, The same,
with the tube twisted in the reverse direction. Devonian, America.
(Onginal.)]
[Illustration: Fig. 88. a b, _Spirorbis laxus_, enlarged, Upper
Silurian, America; c, _Spirorbis spinulifera_, of the natural
size and enlarged, Devonian, Canada. (After Hall and the Author.)]
[Illustration: Fig. 88.--Devonian Trilobites; a, _Phacops latifrons_,
Devonian of Britain, the Continent of Europe, and South America;
b, _Homalonotus armatus_, Europe; c, _Phacops (Trimerocephalus)
loevis_, Europe; d, Head-shield of _Phacops (Portlockia)
granulatus_, Europe. (After Salter and Burmeister.)]
The _Crustaceans_ of the Devonian are principally _Eurypterids_
and _Trilobites_. Some of the former attain gigantic dimensions,
and the quarrymen in the Scotch Old Red give them the name of
"seraphim" from their singular scale-like ornamentation. The
_Trilobites_, though still sufficiently abundant in some localites,
have undergone a yet further diminution since the close of the
Upper Silurian. In both America and Europe quite a number of
generic types have survived from the Silurian, but few or no
new ones make their appearance during this period in either the
Old World or the New. The _species_, however, are distinct; and
the principal forms belong to the genera _Phacops_ (fig. 88, a,
c, d), _Homalonotus_ (fig. 88, b), _Proetus_, and _Bronteus_.
The species figured above under the name of _Phacops latifrons_
(fig. 88, a), has an almost world-wide distribution, being found
in the Devonian of Britain, Belgium, France, Germany, Russia,
Spain, and South America; whilst its place is taken in North
America by the closely-allied _Phacops rana_. In addition to the
_Trilobites_, the Devonian deposits have yielded the remains of a
number of the minute _Ostracoda_, such as _Entomis_ ("_Cypridina_"),
_Leperditia_, &c., which sometimes occur in vast numbers, as
in the so-called "_Cypridina_ Slates" of the German Devonian.
There are also a few forms of _Phyllopods_ (_Estheria_). Taken
as a whole, the Crustacean fauna of the Devonian period presents
many alliances with that of the Upper Silurian, but has only
slight relationships with that of the Lower Carboniferous.
Besides _Crustaceans_, we meet here for the first time with the
remains of _air-breathing Articulates_, in the shape of _Insects_.
So far, these have only been obtained from the Devonian rocks of
North America, and they indicate the existence of at least four
generic types, all more or less allied to the existing May-flies
(_Ephemeridoe_). One of these interesting primitive insects, namely,
_Platephemera antiqua_ (fig. 89), appears to have measured five
inches in expanse of wing; and another (_Xelloneura antiquorum_) has
attached to its wing the remains of a "stridulating-organ" similar
to that possessed by the modern Grasshoppers--the instrument, as
Principal Dawson remarks, of "the first music of living things
that Geology as yet reveals to us."
[Illustration: Fig. 89.--Wing of _Platephemera antiqua_ Devonian,
America. (After Dawson.)]
Amongst the _Mollusca_, the Devonian rocks have yielded a great
number of the remains of Sea-mosses (_Polyzoa_). Some of these
belong to the ancient type _Ptilodictya_, which seems to disappear
here, or to the allied _Clathropora_ (fig. 90), with its fenestrated
and reticulated fronds. We meet also with the graceful and delicate
stems of _Ceriopora_ (fig. 91).
[Illustration: Fig. 90.--Fragment of _Clathropora intertexta_,
of the natural size and enlarged. Devonian, Canada. (Original.)]
[Illustration: Fig. 91.--Fragment of _Ceriopora Hamiltonensis_,
of the natural size and enlarged. Devonian, Canada. (Original.)]
[Illustration: Fig. 92.--Fragment of _Fenestella magnifica_, of
the natural size and enlarged. Devonian, Canada. (Original.)]
[Illustration: Fig. 93.--Fragment of _Retepora Phillipsi_, of
the natural size and enlarged. Devonian, Canada. (Original.)]
[Illustration: Fig. 94.--Fragment of _Fenestella cribrosa_, of
the natural size and enlarged. Dovonian, Canada. (Original.)]
The majority of the Devonian _Polyzoa_ belong, however, to the
great and important Palæozoic group of the Lace-corals (_Fenestella_,
figs. 92 and 94, _Retepora_, fig. 93, _Polypora_, and their allies).
In all these forms there is a horny skeleton, of a fan-like or
funnel-shaped form, which grew attached by its base to some foreign
body. The frond consists of slightly-diverging or nearly parallel
branches, which are either united by delicate cross-bars, or which
bend alternately from side to side, and become directly united
with one another at short intervals--in either case giving origin
to numerous oval or oblong perforations, which communicate to the
whole plant-like colony a characteristic netted and lace-like
appearance. On one of its surfaces--sometimes the internal, sometimes
the external--the frond carries a number of minute chambers or
"cells," which are generally borne in rows on the branches, and
of which each originally contained a minute animal.
[Illustration: Fig. 95.--_Spirifera sculptilis_. Devonian, Canada.
(After Billings.)]
[Illustration: Fig. 96.--_Spirifera mucronata_. Devonian, America.
(After Billings.)]
[Illustration: Fig. 97.--_Atrypa reticularis_. Upper Silurian
and Devonian of Europe and America. (After Billings.)]
The _Brachiopods_ still continue to be represented in great force
through all the Devonian deposits, though not occurring in the
true Old Red Sandstone. Besides such old types as _Orthis,
Strophomena, Lingula, Athyris_, and _Rhynchonella_, we find some
entirely new ones; whilst various types which only commenced their
existence in the Upper Silurian, now undergo a great expansion
and development. This last is especially the case with the two
families of the _Spiriferidoe_ and the _Produclidoe_. The
_Spirifers_, in particular, are especially characteristic of
the Devonian, both in the Old and New Worlds--some of the most
typical forms, such as _Spirifera mucronata_ (fig. 96), having
the shell "winged," or with the lateral angles prolonged to such
an extent as to have earned for them the popular name of
"fossil-butterflies." The closely-allied _Spirifera disjunda_
occurs in Britain, France, Spain, Belgium, Germany, Russia, and
China. The family of the _Productidoe_ commenced to exist in the
Upper Silurian, in the genus _Chonetes_, and we shall hereafter
find it culminating in the Carboniferous in many forms of the great
genus _Producta_[17] itself. In the Devonian period, there is an
intermediate state of things, the genus _Chonetes_ being continued
in new and varied types, and the Carboniferous _Produdoe_ being
represented by many forms of the allied group _Productella_.
Amongst other well-known Devonian Brachiopods may be mentioned
the two long-lived and persistent types _Atrypa reticularis_
(fig. 97) and _Strophomena rhomboidalis_ (fig. 98). The former
of these commences in the Upper Silurian, but is more abundantly
developed in the Devonian, having a geographical range that is
nothing less than world-wide; whilst the latter commences in the
Lower Silurian, and, with an almost equally cosmopolitan range,
survives into the Carboniferous period.
[Footnote 17: The name of this genus is often written _Productus_,
just as _Spirifera_ is often given in the masculine gender as
_Spirifer_ (the name originally given to it). The masculine
termination to these names is, however, grammatically incorrect,
as the feminine noun _cochlea_ (shell) is in these cases
_understood_.]
[Illustration: Fig. 98.--_Strophomena rhomboidalis_. Lower Silurian,
Upper Silurian, and Devonian of Europe and America.]
[Illustration: Fig. 99.--Different views of _Platyceras dumosum_,
of the natural size. Devonian, Canada. (Original.)]
The Bivalves (_Lamellibranchiata_) of the Devonian call for no
special comment, the genera _Pterinea_ and _Megalodon_ being,
perhaps, the most noticeable. The Univalves (_Gasteropods_), also,
need not be discussed in detail, though many interesting forms
of this group are known. The type most abundantly represented,
especially in America, is _Platyceras_ (fig. 99), comprising thin,
wide-mouthed shells, probably most nearly allied to the existing
"Bonnet-limpets," and sometimes attaining very considerable
dimensions. We may also note the continuance of the genus
_Euomphalus_, with its discoidal spiral shell. Amongst the
_Heteropods_, the survival of _Bellerophon_ is to be recorded;
and in the "Winged-snails," or _Pteropods_, we find new forms
of the old genera _Tentaculites_ and _Conularia_ (fig. 100).
The latter, with its fragile, conical, and often beautifully
ornamented shell, is especially noticeable.
[Illustration: Fig. 100.--_Conularia ornata, of the natural size.
Devonian, Europe.]
[Illustration: Fig. 101.--_Clymenia Sedgwickii_. Devonian, Europe.]
The remains of _Cephalopoda_ are far from uncommon in the Devonian
deposits, all the known forms being still Tetrabranchiate. Besides
the ancient types _Orthoceras_ and _Cyrtoceras_, we have now
a predominance of the spirally-coiled chambered shells of
_Goniatites_ and _Clymenia_. In the former of these the shell is
shaped like that of the _Nautilus_; but the partitions between the
chambers ("septa") are more or less lobed, folded, or angulated,
and the "siphuncle" runs along the _back_ or convex side of the
shell--these being characters which approximate _Goniatites_ to
the true Ammonites of the later rocks. In _Clymenia_, on the
other hand, whilst the shell (fig. 101) is coiled into a flat
spiral, and the partitions or septa are simple or only slightly
lobed, there is still this difference, as compared with the
_Nautilus_, that the tube of the siphuncle is placed on the _inner_
or concave side of the shell. The species of _Clymenia_ are
exclusively Devonian in their range; and some of the limestones
of this period in Germany are so richly charged with fossils of
this genus as to have received the name of "Clymenien-kalk."
The sub-kingdom of the _Vertebrates_ is still represented by
_Fishes_ only; but these are so abundant, and belong to such
varied types, that the Devonian period has been appropriately
called the "Age of Fishes." Amongst the existing fishes there are
three great groups which are of special geological importance,
as being more or less extensively represented in past time. These
groups are: (1) The _Bony Fishes_ (_Teleostei_), comprising most
existing fishes, in which the skeleton is more or less completely
converted into bone; the tail is symmetrically lobed or divided
into equal moieties; and the scales are usually thin, horny,
flexible plates, which overlap one another to a greater or less
extent. (2) The _Ganoid Fishes_ (_Ganoidei_), comprising the modern
Gar-pikes, Sturgeons, &c., in which the skeleton usually more or
less completely retains its primitive soft and cartilaginous
condition; the tail is generally markedly unsymmetrical, being
divided into two unequal lobes; and the scales (when present)
have the form of plates of bone, usually covered by a layer of
shining enamel. These scales may overlap; or they may be rhomboidal
plates, placed edge to edge in oblique rows; or they have the form
of large-sized bony plates, which are commonly united in the region
of the head to form a regular buckler. (3) The _Placoid Fishes_,
or _Elasmobranchii_, comprising the Sharks, Rays, and _Chimoeroe_
of the present day, in which the skeleton is cartilaginous; the
tail is unsymmetrically lobed; and the scales have the form of
detached bony plates of variable size, scattered in the integument.
It is to the two last of these groups that the Devonian fishes
belong, and they are more specially referable to the _Ganoids_.
The order of the Ganoid fishes at the present day comprises but
some seven or eight genera, the species of which principally or
exclusively inhabit fresh waters, and all of which are confined
to the northern hemisphere. As compared, therefore, with the Bony
fishes, which constitute the great majority of existing forms,
the Ganoids form but an extremely small and limited group. It was
far otherwise, however, in Devonian times. At this period, the
bony fishes are not known to have come into existence at all, and
the Ganoids held almost undisputed possession of the waters. To
what extent the Devonian Ganoids were confined to fresh waters
remains yet to be proved; and that many of them lived in the sea
is certain. It was formerly supposed that the Old Red Sandstone
of Scotland and Ireland, with its abundant fish-remains, might
perhaps be a fresh-water deposit, since the habitat of its fishes
is uncertain, and it contains no indubitable marine fossils. It
has been now shown, however, that the marine Devonian strata
of Devonshire and the continent of Europe contain some of the
most characteristic of the Old Red Sandstone fishes of Scotland;
whilst the undoubted marine deposit of the Corniferous limestone
of North America contains numerous shark-like and Ganoid fishes,
including such a characteristic Old Red genus as _Coccosleus_.
There can be little doubt, therefore, but that the majority of
the Devonian fishes were truly marine in their habits, though
it is probable that many of them lived in shallow water, in the
immediate neighbourhood of the shore, or in estuaries.
[Illustration: Fig. 102.--Fishes of the Devonian rocks of America.
a, Diagram of the jaws and teeth of _Dinichthys Hertzeri_,
viewed from the front, and greatly reduced; b, Diagram of the
skull of _Macropetalichthys Sullivanti_, reduced in size; c,
A portion of the enamelled surface of the skull of the same,
magnified; d, One of the scales of _Onychodus sigmoides_, of
the natural size; e, One of the front teeth of the lower jaw of
the same, of the natural size: f, Fin-spine of _Machoeracanthus
major_, a shark-like fish, reduced in size. (After Newberry.)]
[Illustration: Fig. 103.--_Cephalaspis Lyellii_. Old Red Sandstone,
Scotland. (After Page.)]
[Illustration: Fig. 104.--_Pterichthys cornutus_. Old Red Sandstone,
Scotland. (After Agassiz.)]
The Devonian Galloids belong to a number of groups; and it is
only possible to notice a few of the most important forms here.
The modern group of the Sturgeons is represented, more or less
remotely, by a few Devonian fishes--such as _Asterosteus_; and
the great _Macropetalichthys_ of the Corniferous limestone of
North America is believed by Newberry to belong to this group. In
this fish (fig. 102, b) the skull was of large size, its outer
surface being covered with a tuberculated enamel; and, as in the
existing Sturgeons, the mouth seems to have been wholly destitute
of teeth. Somewhat allied, also, to the Sturgeons, is a singular
group of armoured fishes, which is highly characteristic of the
Devonian of Britain and Europe, and less so of that of America.
In these curious forms the head and front extremity of the body
were protected by a buckler composed of large enamelled plates,
more or less firmly united to one another; whilst the hinder end
of the body was naked, or was protected with small scales. Some
forms of this group--such as _Pteraspis_ and _Coccosteus_--date
from the Upper Silurian; but they attain their maximum in the
Devonian, and none of them are known to pass upwards into the
overlying Carboniferous rocks. Amongst the most characteristic
forms of this group may be mentioned _Cephalaspis_ (fig. 103) and
_Pterichthys_ (fig. 104). In the former of these the head-shield is
of a crescentic shape, having its hinder angles produced backwards
into long "horns," giving it the shape of a "saddler's knife."
No teeth have been discovered; but the body was covered with
small ganoid scales, and there was an unsymmetrical tail-fin.
In _Pterichthys_--which, like the preceding, was first brought
to light by the labours of Hugh Miller--the whole of the head
and the front part of the body were defended by a buckler of
firmly-united enamelled plates, whilst the rest of the body was
covered with small scales. The form of the "pectoral fins" was
quite unique--these having the shape of two long, curved spines,
somewhat like wings, covered by finely-tuberculated ganoid plates.
All the preceding forms of this group are of small size; but
few fishes, living or extinct, could rival the proportions of
the great _Dinichthys_, referred to this family by Newberry.
In this huge fish (fig. 102, a) the head alone is over three
feet in length, and the body is supposed to have been twenty-five
or thirty feet long. The head was protected by a massive cuirass
of bony plates firmly articulated together, but the hinder end
of the body seems to have been simply enveloped in a leathery
skin. The teeth are of the most formidable description, consisting
in both jaws of serrated dental plates behind, and in front of
enormous conical tusks (fig. 102, a). Though immensely larger,
the teeth of _Dinichthys_ present a curious resemblance to those
of the existing Mud-fishes (_Lepidosiren_).
In another great group of Devonian Ganoids, we meet with fishes
more or less closely allied to the living _Polypteri_ (fig. 105)
of the Nile and Senegal. In this group (fig. 106) the pectoral
fins consist of a central scaly lobe carrying the fin-rays on
both sides, the scales being sometimes rounded and overlapping
(fig. 106), or more commonly rhomboidal and placed edge to edge
(fig. 105, A). Numerous forms of these "Fringe-finned" Ganoids
occur in the Devonian strata, such as _Holoptychius, Glyotoloemus,
Osteolepis, Phaneropleuron_, &c. To this group is also to be
ascribed the huge _Onychodus_ (fig. 102, d and e), with its
large, rounded, overlapping scales, an inch in diameter, and its
powerful pointed teeth. It is to be remembered, however, that
some of these "Fringe-finned" Ganoids are probably referable
to the small but singular group of the "Mud-fishes" (_Dipnoi_),
represented at the present day by the singular _Lepidosiren_
of South America and Africa, and the _Ceratodus_ of the rivers
of Queensland.
[Illustration: Fig. 105.--A, _Polypterus_, a recent Ganoid
fish; B, _Osteolepis_, a Devonian Ganoid; a a, Pectoral fins,
showing the fin-rays arranged round a central lobe.]
[Illusration: Fig. 106.--_Holoptychius nobilissimus_, restored.
Old Red Sandstone, Scotland. A, Scale of the same.]
Leaving the Ganoid fishes, it still remains to be noticed that
the Devonian deposits have yielded the remains of a number of
fishes more or less closely allied to the existing Sharks, Rays,
and _Chimoeroe_ (the _Elasmobranchii_). The majority of the forms
here alluded to are allied not to the true Sharks and Dog-fishes,
but to the more peaceable "Port Jackson Sharks," with their blunt
teeth, adapted for crushing the shells of Molluscs. The collective
name of "Cestracionts" is applied to these; and we have evidence of
their past existence in the Devonian seas both by their teeth, and
by the defensive spines which were implanted in front of a greater
or less number of the fins. These are bony spines, often variously
grooved, serrated, or ornamented, with hollow bases, implanted
in the integument, and capable of being erected or depressed
at will. Many of these "fin-spines" have been preserved to us
in the fossil condition, and the Devonian rocks have yielded
examples belonging to many genera. As some of the true Sharks
and Dog-fishes, some of the Ganoids, and even some Bony Fishes,
possess similar defences, it is often a matter of some uncertainty
to what group a given spine is to be referred. One of these spines,
belonging to the genus _Machoeracanthus_, from the Devonian rocks
of America, has been figured in a previous illustration (fig.
102, f).
In conclusion, a very few words may be said as to the validity of
the Devonian series as an independent system of rocks, preserving
in its successive strata the record of an independent system
of life. Some high authorities have been inclined to the view
that the Devonian formation has in nature no actual existence,
but that it is made up partly of beds which should be referred
to the summit of the Upper Silurian, and partly of beds which
properly belong to the base of the Carboniferous. This view seems
to have been arrived at in consequence of a too exclusive study
of the Devonian series of the British Isles, where the physical
succession is not wholly clear, and where there is a striking
discrepancy between the organic remains of those two members
of the series which are known as the "Old Red Sandstone" and
the "Devonian" rocks proper. This discrepancy, however, is not
complete; and, as we have seen, can be readily explained on the
supposition that the one group of rocks presents us with the
shallow water and littoral deposits of the period, while in the
other we are introduced to the deep-sea accumulations of the
same period. Nor can the problem at issue be solved by an appeal
to the phenomena of the British area alone, be the testimony of
these what it may. As a matter of fact, there is at present no
sufficient ground for believing that there is any irreconcilable
discordance between the succession of rocks and of life in Britain
during the period which elapsed between the deposition of the
Upper Ludlow and the formation of the Carboniferous Limestone,
and the order of the same phenomena during the same period in
other regions. Some of the Devonian types of life, as is the
case with all great formations, have descended unchanged from
older types; others pass upwards unchanged to the succeeding
period: but the fauna and flora of the Devonian period are, as
a whole, quite distinct from those of the preceding Silurian or
the succeeding Carboniferous; and they correspond to an equally
distinct rock-system, which in point of time holds an intermediate
position between the two great groups just mentioned. As before
remarked, this conclusion may be regarded as sufficiently proved
even by the phenomena of the British area; but it maybe said to
be rendered a certainty by the study of the Devonian deposits of
the continent of Europe--or, still more, by the investigation of
the vast, for the most part uninterrupted and continuous series
of sediments which commenced to be laid down in North America
at the beginning of the Upper Silurian, and did not cease till,
at any rate, the close of the Carboniferous.
LITERATURE.
The following list comprises the more important works and memoirs
to which the student of Devonian rocks and fossils may refer:--
(1) 'Siluria.' Sir Roderick Murchison.
(2) 'Geology of Russia in Europe.' Murchison (together with De
Verneuil and Count von Keyserling).
(3) "Classification of the Older Rocks of Devon and Cornwall"--'Proc.
Geol. Soc.,' vol. iii., 1839. Sedgwick and Murchison.
(4) "On the Physical Structure of Devonshire;" and on the
"Classification of the Older Stratified Rocks of Devonshire
and Cornwall"--'Trans. Geol. Soc.,' vol. v., 1840. Sedgwick
and Murchison.
(5) "On the Distribution and Classification of the Older or Palæozoic
Rocks of North Germany and Belgium"--'Geol. Trans.,' 2d ser.,
vol. vi., 1842. Sedgwick and Murchison.
(6) 'Report on the Geology of Cornwall, Devon, and West Somerset.'
De la Beche.
(7) 'Memoirs of the Geological Survey of Ireland and Scotland.'
Jukes and Geikie.
(8) "On the Carboniferous Slate (or Devonian Rocks) and the Old
Red Sandstone of South Ireland and North Devon"--'Quart.
Journ. Geol. Soc.,' vol. xxii. Jukes.
(9) "On the Physical Structure of West Somerset and North Devon;"
and on the "Palæontological Value of Devonian Fossils"--'Quart.
Journ. Geol. Soc.,' vol. iii. Etheridge.
(10) "On the Connection of the Lower, Middle, and Upper Old Red
Sandstone of Scotland"--'Trans. Edin. Geol. Soc.,' vol. i.
part ii. Powrie.
(11) 'The Old Red Sandstone,' 'The Testimony of the Rocks,' and
'Footprints of the Creator.' Hugh Miller.
(12) "Report on the 4th Geological District"--'Geology of New York,'
vol. iv. James Hall.
(13) 'Geology of Canada,' 1863. Sir W. E. Logan.
(14) 'Acadian Geology.' Dawson.
(15) 'Manual of Geology.' Dana.
(16) 'Geological Survey of Ohio,' vol. i.
(17) 'Geological Survey of Illinois,' vol. i.
(18) 'Palæozoic Fossils of Cornwall, Devon, and West Somerset.'
Phillips.
(19) 'Recherches sur les Poissons Fossiles.' Agassiz.
(20) 'Poissous de l'Old Red.' Agassiz.
(21) "On the Classification of Devonian Fishes"--' Mem. Geol. Survey
of Great Britain,' Decade X. Huxley.
(22) 'Monograph of the Fishes of the Old Red Sandstone of Britain'
(Palæontographical Society). Powrie and Lankester.
(23) 'Fishes of the Devonian System, Palæontology of Ohio.' Newberry.
(24) 'Monograph of British Trilobites' (Palæontographical Society);
Salter.
(25) 'Monograph of British Merostomata' (Palæontographical Society).
Henry Woodward.
(26) 'Monograph of British Brachiopoda' (Palæontographical Society).
Davidson.
(27) 'Monograph of British Fossil Corals' (Palæontographical Society).
Milne-Edwards and Haime.
(28) 'Polypiers Foss. des Terrains Paléozoiques.' Milne-Edwards
and Jules Haime.
(29) "Devonian Fossils of Canada West"--'Canadian Journal,' new ser.,
vols. iv.-vi. Billings.
(30) 'Palæontology of New York,' vol. iv. James Hall.
(31) 'Thirteenth, Fifteenth, and Twenty-third Annual Reports on the
State Cabinet.' James Hall.
(32) 'Palæozoic Fossils of Canada,' vol. ii. Billings.
(33) 'Reports on the Palæontology of the Province of Ontario for 1874
and 1875.' Nicholson.
(34) "The Fossil Plants of the Devonian and Upper Silurian Formations
of Canada"--'Geol. Survey of Canada.' Dawson.
(35) 'Petrefacta Germaniæ.' Goldfuss.
(36) 'Versteinerungen der Grauwacken-formation.' &c. Geinitz.
(37) 'Beitrag zur Palæontologie des Thüringer-Waldes.' Richter and
Unger.
(38) 'Ueber die Placodermen der Devonischen System.' Pander.
(39) 'Die Gattungen der Fossilen Pflanzen.' Goeppert.
(40) 'Genera et Species Plantarum Fossilium.' Unger.