THE LOWER SILURIAN PERIOD.
The great system of deposits to which Sir Roderick Murchison
applied the name of "Silurian Rocks" reposes directly upon the
highest Cambrian beds, apparently without any marked unconformity,
though with a considerable change in the nature of the fossils. The
name "Silurian" was originally proposed by the eminent geologist
just alluded to for a great series of strata lying below the Old
Red Sandstone, and occupying districts in Wales and its borders
which were at one time inhabited by the "Silures," a tribe of
ancient Britons. Deposits of a corresponding age are now known
to be largely developed in other parts of England, in Scotland,
and in Ireland, in North America, in Australia, in India, in
Bohemia, Saxony, Bavaria, Russia, Sweden and Norway, Spain, and
in various other regions of less note. In some regions, as in
the neighbourhood of St Petersburg, the Silurian strata are found
not only to have preserved their original horizontality, but
also to have retained almost unaltered their primitive soft and
incoherent nature. In other regions, as in Scandinavia and many
parts of North America, similar strata, now consolidated into
shales, sandstones, and limestones, may be found resting with
a very slight inclination on still older sediments. In a great
many regions, however, the Silurian deposits are found to have
undergone more or less folding, crumpling, and dislocation,
accompanied by induration and "cleavage" of the finer and softer
sediments; whilst in some regions, as in the Highlands of Scotland,
actual "metamorphism" has taken place. In consequence of the
above, Silurian districts usually present the bold, rugged, and
picturesque outlines which are characteristic of the older
"Primitive" rocks of the earth's crust in general. In many instances,
we find Silurian strata rising into mountain-chains of great
grandeur and sublimity, exhibiting the utmost diversity of which
rock-scenery is capable, and delighting the artist with endless
changes of valley, lake, and cliff. Such districts are little
suitable for agriculture, though this is often compensated for
by the valuable mineral products contained in the rocks. On the
other hand, when the rocks are tolerably soft and uniform in
their nature, or when few disturbances of the crust of the earth
have taken place, we may find Silurian areas to be covered with
an abundant pasturage or to be heavily timbered.
Under the head of "Silurian Rocks," Sir Roderick Murchison included
all the strata between the summit of the "Longmynd." beds and the
Old Red Sandstone, and he divided these into the two great groups
of the _Lower_ Silurian and _Upper_ Silurian. It is, however, now
generally admitted that a considerable portion of the basement
beds of Murchison's Silurian series must be transferred---if only
upon palæontological grounds--to the Upper Cambrian, as has here
been done; and much controversy has been carried on as to the proper
nomenclature of the Upper Silurian and of the remaining portion
of Murchison's Lower Silurian. Thus, some would confine the name
"Silurian" exclusively to the Upper Silurian, and would apply the
name of "Cambro-Silurian" to the Lower Silurian, or would include
all beds of the latter age in the "Cambrian" series of Sedgwick.
It is not necessary to enter into the merits of these conflicting
views. For our present purpose, it is sufficient to recognise
that there exist two great groups of rocks between the highest
Cambrian beds, as here defined, and the base of the Devonian or
Old Red Sandstone. These two great groups are so closely allied
to one another, both physically and palæontologically, that many
authorities have established a third or intermediate group (the
"Middle Silurian"), by which a passage is made from one into
the other. This method of procedure involves disadvantages which
appear to outweigh its advantages; and the two groups in question
are not only generally capable of very distinct stratigraphical
separation, but at the same time exhibit, together with the alliances
above spoken of, so many and such important palæontological
differences, that it is best to consider them separately. We
shall therefore follow this course in the present instance; and
pending the final solution of the controversy as to Cambrian and
Silurian nomenclature, we shall distinguish these two groups
of strata as the "Lower Silurian" and the "Upper Silurian."
The _Lower Silurian Rocks_ are known already to be developed
in various regions; and though their _general_ succession in
these areas is approximately the same, each area exhibits
peculiarities of its own, whilst the subdivisions of each are
known by special names. All, therefore, that can be attempted
here, is to select two typical areas--such as Wales and North
America and to briefly consider the grouping and divisions of
the Lower Silurian in each.
In Wales, the line between the Cambrian and Lower Silurian is
somewhat ill-defined, and is certainly not marked by any strong
unconformity. There are, however; grounds for accepting the line
proposed, for palæontological reasons, by Dr Hicks, in accordance
with which the Tremadoc Slates ("Lower Tremadoc" of Salter) become
the highest of the Cambrian deposits of Britain. If we take this
view, the Lower Silurian rocks of Wales and adjoining districts
are found to have the following _general_ succession from below
upwards (fig. 34):--
1. The _Arenig Group_.--This group derives its name from the
Arenig mountains, where it is extensively developed. It consists
of about 4000 feet of slates, shales, and flags, and is divisible
into a lower, middle, and upper division, of which the former
is often regarded as Cambrian under the name of "Upper Tremadoc
Slates."
2. The _Llandeilo Group_.--The thickness of this group varies
from about 4000 to as much as 10,000 feet; but in this latter
case a great amount of the thickness is made up of volcanic ashes
and interbedded traps. The sedimentary beds of this group are
principally slates and flags, the latter occasionally with calcareous
bands; and the whole series can be divided into a lower, middle,
and upper Llandeilo division, of which the last is the most
important. The name of "Llandeilo" is derived from the town of
the same name in Wales, where strata of this age were described
by Murchison.
3. The _Caradoc_ or _Bala Group_.--The alternative names of this
group are also of local origin, and are derived, the one from
Caer Caradoc in Shropshire, the other from Bala in Wales, strata
of this age occurring in both localities. The series is divided
into a lower and upper group, the latter chiefly composed of
shales and flags, and the former of sandstones and shales, together
with the important and interesting calcareous band known as the
"Bala Limestone." The thickness of the entire series varies from
4000 to as much as 12,000 feet, according as it contains more
or less of interstratified igneous rocks.
4. The _Llandovery Group_ (Lower Llandovery of Murchison).--This
series, as developed near the town of Llandovery, in
Caermarthenshire, consists of less than 1000 feet of conglomerates,
sandstones, and shales. It is probable, however, that the little
calcareous band known as the "Hirnant Limestone," together with
certain pale-coloured slates which lie above the Bala Limestone,
though usually referred to the Caradoc series, should in reality
be regarded as belonging to the Llandovery group.
The general succession of the Lower Silurian strata of Wales
and its borders, attaining a maximum thickness (along with
contemporaneous igneous matter) of nearly 30,000 feet, is
diagramatically represented in the annexed sketch-section (fig.
34):--
[Illustration: Fig 34. GENERALIZED SECTION OF THE LOWER SILURIAN
ROCKS OF WALES.]
In North America, both in the United States and in Canada, the
Silurian rocks are very largely developed, and may be regarded
as constituting an exceedingly full and typical series of the
deposits of this period. The chief groups of the Silurian rocks
of North America are as follows, beginning, as before, with the
lowest strata, and proceeding upwards (fig. 35):--
1. _Quebec Group_.--This group is typically developed in the
vicinity of Quebec, where it consists of about 5000 feet of strata,
chiefly variously-coloured shales, together with some sandstones
and a few calcareous bands. It contains a number of peculiar
Graptolites, by which it can be identified without question with
the Arenig group of Wales and the corresponding Skiddaw Slates
of the North of England. It is also to be noted that numerous
Trilobites of a distinct Cambrian _facies_ have been obtained in
the limestones of the Quebec group, near Quebec. These fossils,
however, have been exclusively obtained from the limestones of
the group; and as these limestones are principally calcareous
breccias or conglomerates, there is room for believing that these
primordial fossils are really derived, in part at any rate, from
fragments of an upper Cambrian limestone. In the State of New
York, the Graptolitic shales of Quebec are wanting; and the base
of the Silurian is constituted by the so-called "Calciferous
Sand-rock" and "Chazy Limestone."[11] The first of these is
essentially and typically calcareous, and the second is a genuine
limestone.
[Footnote 11: The precise relations of the Quebec shales with
Graptolites (Levis Formation) to the Calciferous and Chazy beds
are still obscure, though there seems little doubt but that the
Quebec Shales are superior to the Calciferous Sand-rock.]
2. The _Trenton Group_.--This is an essentially calcareous group,
the various limestones of which it is composed being known as
the "Bird's-eye," "Black River," and "Trenton" Limestones, of
which the last is the thickest and most important. The thickness
of this group is variable, and the bands of limestone in it are
often separated by beds of shale.
3. The _Cincinnati Group_ (Hudson River Formation[12]).--This
group consists essentially of a lower series of shales, often
black in colour and highly charged with bituminous matter (the
"Utica Slates "), and of an upper series of shales, sandstones, and
limestones (the "Cincinnati" rocks proper). The exact parallelism
of the Trenton and Cincinnati groups with the subdivisions of the
Welsh Silurian series can hardly be stated positively. Probably
no precise equivalency exists; but there can be no doubt but that
the Trenton and Cincinnati groups correspond, as a whole, with the
Llandeilo and Caradoc groups of Britain. The subjoined diagrammatic
section (fig. 35) gives a general idea of the succession of the
Lower Silurian rocks of North America:--
[Illustration: Fig 35. GENERALIZED SECTION OF THE LOWER SILURIAN
ROCKS OF NORTH AMERICA.]
[Illustration: Fig. 36.--_Licrophycus Ottawaensis_ a "Fucoid,"
from the Trenton Limestone (Lower Silurian) of Canada. (After
Billings.)]
[Footnote 12: There is some difficulty about the precise nomenclature
of this group. It was originally called the "Hudson River Formation;"
but this name is inappropriate, as rocks of this age hardly touch
anywhere the actual Hudson River itself, the rocks so called
formerly being now known to be of more ancient date. There is
also some want of propriety in the name of "Cincinnati Group,"
since the rocks which are known under this name in the vicinity of
Cincinnati itself are the representatives of the Trenton Limestone,
Utica Slates, and the old Hudson River group, inseparably united
in what used to be called the "Blue Limestone Series."].
Of the _life_ of the Lower Silurian period we have record in
a vast number of fossils, showing that the seas of this period
were abundantly furnished with living denizens. We have, however,
in the meanwhile, no knowledge of the land-surfaces of the period.
We have therefore no means of speculating as to the nature of
the terrestrial animals of this ancient age, nor is anything
known with certainty of any land-plants which may have existed.
The only relics of vegetation upon which a positive opinion can
be expressed belong to the obscure group of the "Fucoids," and
are supposed to be the remains of sea-weeds. Some of the fossils
usually placed under this head are probably not of a vegetable
nature at all, but others (fig. 36) appear to be unquestionable
plants. The true affinities of these, however, are extremely
dubious. All that can be said is, that remains which appear to
be certainly vegetable, and which are most probably due to marine
plants, have been recognised nearly at the base of the Lower
Silurian (Arenig), and that they are found throughout the series
whenever suitable conditions recur.
The Protozoans appear to have flourished extensively in the Lower
Silurian seas, though to a large extent under forms which are
still little understood. We have here for the first time the
appearance of Foraminifera of the ordinary type--one of the most
interesting observations in this collection being that made by
Ehrenberg, who showed that the Lower Silurian sandstones of the
neighbourhood of St Petersburg contained casts in glauconite of
Foraminiferous shells, some of which are referable to the existing
genera _Rotalia_ and _Texularia_. True _Sponges_, belonging to
that section of the group in which the skeleton is calcareous,
are also not unknown, one of the most characteristic genera being
_Astylospongia_ (fig. 37). In this genus are included more or
less globular, often lobed sponges, which are believed not to
have been attached to foreign bodies. In the form here figured
there is a funnel-shaped cavity at the summit; and the entire
mass of the sponge is perforated, as in living examples, by a
system of canals which convey the sea-water to all parts of the
organism. The canals by which the sea-water gains entrance open
on the exterior of the sphere, and those by which it again escapes
from the sponge open into the cup-shaped depression at the summit.
[Illustration: Fig. 37.--_Astylospongia proemorsa_, cut vertically
so as to exhibit the canal-system in the interior. Lower Silurian,
Tennessee. (After Ferdinand Roemer.)]
The most abundant, and at the same time the least understood,
of Lower Silurian Protozoans belong, however, to the genera
_Stromatopora_ and _Receptaculites_, the structure of which can
merely be alluded to here. The specimens of _Stromatopora_ (fig.
38) occur as hemispherical, pear-shaped, globular, or irregular
masses, often of very considerable size, and sometimes demonstrably
attached to foreign bodies. In their structure these masses consist
of numerous thin calcareous laminæ, usually arranged concentrically,
and separated by narrow interspaces. These interspaces are generally
crossed by numerous vertical calcareous pillars, giving the vertical
section of the fossil a lattice-like appearance. There are also
usually minute pores in the concentric laminæ, by which the
successive interspaces are placed in communication; and sometimes
the surface presents large rounded openings, which appear to
correspond with the water-canals of the Sponges. Upon the whole,
though presenting some curious affinities to the calcareous Sponges,
_Stromatopora_ is perhaps more properly regarded as a gigantic
_Foraminifer_. If this view be correct, it is of special interest
as being probably the nearest ally of _Eozoön_, the general
appearance of the two being strikingly similar, though their
minute structure is not at all the same. Lastly, in the fossils
known as _Receptaculites_ and _Ischadites_ we are also presented
with certain singular Lower Silurian Protozoans, which may with
great probability be regarded as gigantic _Foraminifera_. Their
structure is very complex; but fragments are easily recognised
by the fact that the exterior is covered with numerous rhomboidal
calcareous plates, closely fitting together, and arranged in
peculiar intersecting curves, presenting very much the appearance
of the engine-turned case of a watch.
[Illustration: Fig. 38.--A small and perfect specimen of
_Stromatopora rugosa_, of the natural size, from the Trenton
Limestone of Canada. (After Billings.)]
Passing next to the sub-kingdom of _Coelenterate_ animals (Zoophytes,
Corals, &c.), we find that this great group, almost or wholly
absent in the Cambrian, is represented in Lower Silurian deposits
by a great number of forms belonging on the one hand to the true
Corals, and en the other hand to the singular family of the
_Graptolites_. If we except certain plant-like fossils which
probably belong rather to the Sertularians or the Polyzoans (e.g.,
_Dictyonema, Dendrograptus_, &c.), the family of the _Graptolites_
may be regarded as exclusively Silurian in its distribution. Not
only is this the case, but it attained its maximum development
almost upon its first appearance, in the Arenig Rocks; and whilst
represented by a great variety of types in the Lower Silurian;
it only exists in the Upper Silurian in a much diminished form.
The _Graptolites_ (Gr. _grapho_, I write; _lithos_, stone) were
so named by Linnæus, from the resemblance of some of them to
written or pencilled marks upon the stone, though the great
naturalist himself did not believe them to be true fossils at
all. They occur as linear or leaf-like bodies, sometimes simple,
sometimes compound and branched; and no doubt whatever can be
entertained as to their being the skeletons of composite organisms,
or colonies of semi-independent animals united together by a common
fleshy trunk, similar to what is observed in the colonies of the
existing Sea-firs (Sertularians). This fleshy trunk or common
stem of the colony was protected by a delicate horny sheath, and
it gave origin to the little flower-like "polypites," which
constituted the active element of the whole assemblage. These
semi-independent beings were, in turn, protected each by a little
horny cup or cell, directly connected with the common sheath
below, and terminating above in an opening through which the
polypite could protrude its tentacled head or could again withdraw
itself for safety. The entire skeleton, again, was usually, if
not universally, supported by a delicate horny rod or "axis,"
which appears to have been hollow, and which often protrudes to
a greater or less extent beyond one or both of the extremities
of the actual colony.
The above gives the elementary constitution of any _Graptolite_,
but there are considerable differences as to the manner in which
these elements are arranged and combined. In some forms the common
stem of the colony gives origin to but a single row of cells
on one side. If the common stem is a simple, straight, or
slightly-curved linear body, then we have the simplest form of
Graptolite known (the genus _Monograptus_); and it is worthy of
note that these simple types do not come into existence till
comparatively late (Llandeilo), and last nearly to the very close
of the Upper Silurian. In other cases, whilst there is still but
a single row of cells, the colony may consist of two of these
simple stems springing from a common point, as in the so-called
"twin Graptolites" (_Didymograptus_, fig. 40). This type is entirely
confined to the earlier portion of the Lower Silurian period
(Arenig and Llandeilo). In other cases, again, there may be four
of such stems springing from a central point (_Tetragraptus_).
Lastly, there are numerous complex forms (such as _Dichograptus,
Loganograptus_, &c.) in which there are eight or more of these
simple branches, all arising from a common centre (fig. 39),
which is sometimes furnished with a singular horny disc. These
complicated branching forms, as well as the _Tetragrapti_, are
characteristic of the horizon of the Arenig group. Similar forms,
often specifically identical, are found at this horizon in Wales,
in the great series of the Skiddaw Slates of the north of England,
in the Quebec group in Canada, in equivalent beds in Sweden, and
in certain gold-bearing slates of the same age in Victoria in
Australia.
[Illustration: Fig. 39.--_Dichograptus octobrachiatus_, a branched,
"unicellular" Graptolite from the Skiddaw and Quebec Groups (Arenig).
(After Hall.)]
In another great group of Graptolites (including the genera
_Diplograptus, Dicranograptus, Climacograptus_, &c.) the common
stem of the colony gives origin, over part or the whole or its
length, to _two_ rows of cells, one on each side (fig. 41). These
"double-celled" Graptolites are highly characteristic of the Lower
Silurian deposits; and, with an exception more apparent than real
in Bohemia, they are exclusively confined to strata of Lower
Silurian age, and are not known to occur in the Upper Silurian.
Lastly, there is a group of Graptolites (_Phyllograptus_, fig.
42) in which the colony is leaf-like in form, and is composed
of _four_ rows of cells springing in a cross-like manner from
the common stem. These forms are highly characteristic of the
Arenig group.
[Illustration: Fig. 40.--Central portion of the colony of
_Didymegraptus divaricatus_, Upper Llandeilo, Dumfresshire.
(Original.)]
[Illustration: Fig. 41.--Examples of _Diplograptus pristis_,
showing variations in the appendages at the base. Upper Llandeilo,
Dumfriesshire. (Original.)]
[Illustration: Fig. 42.--Group of individuals of _Phyllograptus
typus_, from the Quebec group of Canada. (After Hall.) One of
the four rows of cells is hidden on the under surface.]
The Graptolites are usually found in dark-coloured, often black
shales, which sometimes contain so much carbon as to become
"anthracitic." They may be simply carbonaceous; but they are
more commonly converted into iron-pyrites, when they glitter
with the brilliant lustre of silver as they lie scattered on the
surface of the rock, fully deserving in their metallic tracery
the name of "written stones." They constitute one of the most
important groups of Silurian fossils, and are of the greatest
value in determining the precise stratigraphical position of
the beds in which they occur. They present, however, special
difficulties in their study; and it is still a moot point as
to their precise position in the zoological scale. The balance
of evidence is in favour of regarding them as an ancient and
peculiar group of the Sea-firs (Hydroid Zoophytes), but some
regard them as belonging rather to the Sea-mosses (_Polyzoa_).
Under any circumstances, they cannot be directly compared either
with the ordinary Sea-firs or the ordinary Sea-mosses; for these
two groups consist of fixed organisms, whereas the Graptolites
were certainly free-floating creatures, living at large in the
open sea. The only Hydroid Zoophytes or Polyzoans which have
a similar free mode of existence, have either no skeleton at
all, or have hard structures quite unlike the horny sheaths of
the Graptolites.
The second great group of Coelenterate animals (_Actinozoa_)
is represented in the Lower Silurian rocks by numerous Corals.
These, for obvious reasons, are much more abundant in regions
where the Lower Silurian series is largely calcareous (as in
North America) than in districts like Wales, where limestones
are very feebly developed. The Lower Silurian Corals, though
the first of their class, and presenting certain peculiarities,
may be regarded as essentially similar in nature to existing
Corals. These, as is well known, are the calcareous skeletons of
animals--the so-called "Coral-Zoophytes"--closely allied to the
common Sea-anemones in structure and habit. A _simple_ coral (fig.
43) consists of a calcareous cup embedded in the soft tissues of
the flower-like polype, and having at its summit a more or less
deep depression (the "calice") in which the digestive organs
are contained. The space within the coral is divided into
compartments by numerous vertical calcareous plates (the "septa"),
which spring from the inside of the wall of the cup, and of which
some generally reach the centre. _Compound_ corals, again (fig.
44), consist of a greater or less number of structures similar
in structure to the above, but united together in different ways
into a common mass. _Simple_ corals, therefore, are the skeletons
of _single_ and independent polypes; whilst _compound_ corals
are the skeletons of assemblages or colonies of similar polypes,
living united with one another another as an organic community.
[Illustration: Fig. 43.--_Zaphrentis Stokesi_, a simple "cup-coral,"
Upper Silurian, Canada. (After Billings.)]
[Illustration: Fig. 44.--Upper surface of a mass of _Strombodes
pentagonus_. Upper Silurian, Canada. (After Billings.)]
In the general details of their structure, the Lower Silurian
Corals do not differ from the ordinary Corals of the present
day. The latter, however, have the vertical calcareous plates
of the coral ("septa") arranged in multiples of six or five;
whereas the former have these structures arranged in multiples
of four, and often showing a cross-like disposition. For this
reason, the common Lower Silurian Corals are separated to form
a distinct group under the name of _Rugose_ Corals or _Rugosa_.
They are further distinguished by the fact that the cavity of
the coral ("visceral chamber") is usually subdivided by more
or less numerous horizontal calcareous plates or partitions,
which divide the coral into so many tiers or storeys, and which
are known as the "tabulæ" (fig. 45).
[Illustration: Fig. 45.--_Columnaria alveolata_, a Rugose compound
coral, with imperfect septa, but having the corallites partitioned
off into storeys by "tabulæ." Lower Silurian, Canada. (After
Billings.)]
In addition to the Rugose Corals, the Lower Silurian rocks contain
a number of curious compound corals, the tubes of which have
either no septa at all or merely rudimentary ones, but which
have the transverse partitions or "tabulæ" very highly developed.
These are known as the _Tabulate Corals_; and recent researches
on some of their existing allies (such as _Heliopora_) have shown
that they are really allied to the modern Sea-pens, Organ-pipe
Corals, and Red Coral, rather than to the typical stony Corals.
Amongst the characteristic Rugose Corals of the Lower Silurian
may be mentioned species belonging to the genera _Columnaria,
Favistella, Streptelasma_, and _Zaphrentis_; whilst amongst the
"Tabulate" Corals, the principal forms belong to the genera
_Choetetes, Halysites_ (the Chain-coral), _Constellaria_, and
_Heliolites_. These groups of the Corals, however, attain a greater
development at a later period, and they will be noticed more
particularly hereafter.
[Illustration: Fig. 46.--Group of Cystideans. A, _Caryocrinus
ornatus_,[13] Upper Silurian, America; B, _Pleurocystites squamosus_,
showing two short "arms," Lower Silurian, Canada; C, _Pseudocrinus
bifasciatus_, Upper Silurian, England; D, _Lepadocrinus Gebhartii_,
Upper Silurian, America. (After Hall, Billings, and Salter.)]
[Footnote 13: The genus _Caryocrinus_ is sometimes regarded as
properly belonging to the _Crinoids_, but there seem to be good
reasons for rather considering it as an abnormal form of
_Cystidean_.]
Passing onto higher animals, we find that the class of the
_Echinodermata_ is represented by examples of the Star-fishes
(_Asteroidea_), the Sea-lilies (_Crinoidea_), and the peculiar
extinct group of the Cystideans (_Cystoidea_), with one or two of
the Brittle-stars (_Ophiuroidea_)--the Sea-urchins (_Echinoidea_)
being still wanting. The Crinoids, though in some places extremely
numerous, have not the varied development that they possess in
the Upper Silurian, in connection with which their structure will
be more fully spoken of. In the meanwhile, it is sufficient to
note that many of the calcareous deposits of the Lower Silurian
are strictly entitled to the name of "Crinoidal limestones,"
being composed in great part of the detached joints, and plates,
and broken stems, of these beautiful but fragile organisms (see
fig. 12). Allied to the Crinoids are the singular creatures which
are known as _Cystideans_ (fig. 46). These are generally composed
of a globular or ovate body (the "calyx"), supported upon a short
stalk (the "column"), by which the organism was usually attached
to some foreign body. The body was enclosed by closely-fitting
calcareous plates, accurately jointed together; and the stem was
made up of numerous distinct pieces or joints, flexibly united
to each other by membrane. The chief distinction which strikes
one in comparing the Cystideans with the Crinoids is, that the
latter are always furnished, as will be subsequently seen, with
a beautiful crown of branched and feathery appendages, springing
from the summit of the calyx, and which are composed of innumerable
calcareous plates or joints, and are known as the "arms." In the
Cystideans, on the other hand, there are either no "arms" at all,
or merely short, unbranched, rudimentary arms. The Cystideans are
principally, and indeed nearly exclusively, Silurian fossils;
and though occurring in the Upper Silurian in no small numbers,
they are pre-eminently characteristic of the Llandeilo-Caradoc
period of Lower Silurian time. They commenced their existence,
so far as known, in the Upper Cambrian; and though examples are
not absolutely unknown in later periods, they are pre-eminently
characteristic of the earlier portion of the Palæozoic epoch.
[Illustration: Fig. 47.--Lower Silurian Crustaceans. a, _Asaphus
tyrannus_, Upper Llandeilo; b. _Ogygia Buchii_, Upper Llandeilo;
c, _Trinucleus concentricus_, Caradoc; d, _Caryocaris Wrightii_,
Arenig (Skiddaw Slates); e, _Beyrichia complicata_, natural size and
enlarged, Upper Llandeilo and Caradoc; f, _Primitia strangulata_,
Caradoc: g. Head-shield of _Calymene Blumenbachii_, var.
_brevicapitata_, Caradoc; h, Head-shield of _Triarthrus Becki_
(Utica Slates), United States: i, Shield of _Leperditia
Canadensis_, var. _Josephiana_, of the natural size, Trenton
Limestone, Canada; j, The same, viewed from the front. (After
Salter, M'Coy, Rupert Jones, and Dana.)]
The Ringed Worms (_Annelides_) are abundantly represented in the
Lower Silurian, but principally by tracks and burrows similar
in essential respects to those which occur so commonly in the
Cambrian formation, and calling for no special comment. Much more
important are the _Articulate_ animals, represented as heretofore,
wholly by the remains of the aquatic group of the _Crustaceans_.
Amongst these are numerous little bivalved forms--such as species
of _Primitia_ (fig. 47, f), _Beyrichia_ (fig. 47, e), and
_Leperditia_ (fig. 47, i and j). Most of these are very small,
varying from the size of a pin's head up to that of a hemp seed;
but they are sometimes as large as a small bean (fig. 47, i),
and they are commonly found in myriads together in the rock. As
before said, they belong to the same great group as the living
Water-fleas (_Ostracoda_). Besides these, we find the pod-shaped
head-shields of the shrimp-like Phyllopods--such as _Caryocaris_
(fig. 47, d) and _Ceratiocaris_. More important, however, than
any of these are the _Trilobites_, which may be considered as
attaining their maximum development in the Lower Silurian. The
huge _Paradoxides_ of the Cambrian have now disappeared, and with
them almost all the principal and characteristic "primordial"
genera, save _Olenus_ and _Agnostus_. In their place we have a
great number of new forms--some of them, like the great _Asaphus
tyrannus_ of the Upper Llandeilo (fig. 47, a), attaining a
length of a foot or more, and thus hardly yielding in the matter
of size to their ancient rivals. Almost every subdivision of the
Lower Silurian series has its own special and characteristic
species of Trilobites; and the study of these is therefore of
great importance to the geologist. A few widely-dispersed and
characteristic species have been here figured (fig. 47); and
the following may be considered as the principal Lower Silurian
genera--_Asaphus, Ogygia, Cheirurus, Ampyx, Caiymene, Trinucleus,
Lichas, Illoenus, Æglina, Harpes, Remopleurides, Phacops, Acidaspis_,
and _Homalonotus_, a few of them passing upwards under new forms
into the Upper Silurian.
Coming next to the _Mollusca_, we find the group of the Sea-mosses
and Sea-mats (_Polyzoa_) represented now by quite a number of forms.
Amongst these are examples of the true Lace-corals (_Retepora_
and _Fenestella_), with their netted fan-like or funnel-shaped
fronds; and along with these are numerous delicate encrusting
forms, which grew parasitically attached to shells and corals
(_Hippothoa, Alecto_, &c.); but perhaps the most characteristic
forms belong to the genus _Ptilodictya_ (figs. 48 and 49). In
this group the frond is flattened, with thin striated edges,
sometimes sword-like or scimitar-shaped, but often more or less
branched; and it consists of two layers of cells, separated by
a delicate membrane, and opening upon opposite sides. Each of
these little chambers or "cells" was originally tenanted by a
minute animal, and the whole thus constituted a compound organism
or colony.
[Illustration: Fig. 48.--_Ptilodictya falciformis_. a, Small
specimen of the natural size; b, Cross-section, showing the
shape of the frond; c, Portion of the surface, enlarged. Trenton
Limestone and Cincinnati Group, America. (Original.)]
[Illustration: Fig. 49.--A, _Ptilodictya acuta_; B, _Ptilodictya
Schafferi_. a, Fragment, of the natural size; b, Portion,
enlarged to show the cells. Cincinnati Group of Ohio and Canada.
(Original.)]
[Illustration: Fig. 50.--Lower Silurian Brachiopods. a and
a', _Orthis biforata_, Llandeilo-Caradoc, Britain and America:
b, _Orthis flabellulum_, Caradoc, Britain: c, _Orthis subquadrata_,
Cincinnati Group, America; c', Interior of the dorsal valve of
the same: d, _Strophomena deltoidea_, Llandeilo-Caradoc, Britain
and America. (After Meek, Hall, and Salter.)]
The Lamp-shells or _Brachiopods_ are so numerous, and present
such varied types, both in this and the succeeding period of
the Upper Silurian, that the name of "Age of Brachiopods" has
with justice been applied to the Silurian period as a whole. It
would be impossible here to enter into details as to the many
different forms of Brachiopods which present themselves in the
Lower Silurian deposits; but we may select the three genera _Orthis,
Strophomena_, and _Leptoena_ for illustration, as being specially
characteristic of this period, though not exclusively confined to it.
The numerous shells which belong to the extensive and cosmopolitan
genus _Orthis_ (fig. 50, a, b, c, and fig. 51, c and d),
are usually more or less transversely-oblong or subquadrate, the
two valves (as more or less in all the Brachiopods) of unequal
sizes, generally more or less convex, and marked with radiating
ribs or lines. The valves of the shell are united to one another
by teeth and sockets, and there is a straight hinge-line. The beaks
are also separated by a distinct space ("hinge-area"), formed in
part by each valve, which is perforated by a triangular opening,
through which, in the living condition, passed a muscular cord
attaching the shell to some foreign object. The genus _Strophomena_
(fig. 50, d, and 51, a and b) is very like _Orthis_ in
general character; but the shell is usually much flatter, one
or other valve often being concave, the hinge-line is longer,
and the aperture for the emission of the stalk of attachment is
partially closed by a calcareous plate. In _Leptoena_, again
(fig. 51, e), the shell is like _Strophomena_ in many respects,
but generally comparatively longer, often completely semicircular,
and having one valve convex and the other valve concave. Amongst
other genera of Brachiopods which are largely represented in the
Lower Silurian rocks may be mentioned _Lingula, Crania, Discina,
Trematis, Siphonotreta, Acrotreta, Rhynchonella_, and _Athyris_;
but none of these can claim the importance to which the three
previously-mentioned groups are entitled.
[Illustration: Fig. 51.--Lower Silurian Brachiopods, a, _Strophomena
alternata_, Cincinnati Group, America; b, _Strophomena filitexta,
Trenton and Cincinnati Groups, America; c, _Orthis testudinaria_,
Caradoc, Europe, and America; d, d', _Orthis plicateila_, Cincinnati
Group, America; e, e', e'', _Leptoena sericea_, Llandeilo and
Caradoc, Europe and America. (After Meek, Hall, and the Author.)]
The remaining Lower Silurian groups of _Mollusca_ can be but
briefly glanced at here. The Bivalves (_Lamellibranchiata_) find
numerous representatives, belonging to such genera as _Modiolopsis,
Ctenodonta, Orthonota, Paloearca, Lyrodesma, Ambonychia_, and
_Cleidophorus_. The Univalves (_Gasteropoda_) are also very numerous,
the two most important genera being _Murchisonia_ (fig. 52) and
_Pleurotomaria_. In both these groups the outer lip of the shell
is notched; but the shell in the former is elongated and turreted,
whilst in the latter it is depressed. The curious oceanic Univalves
known as the _Heteropods_ are also very abundant, the principal
forms belonging to _Bellerophon_ and _Maclurea_. In the former
(fig. 53) there is a symmetrical convoluted shell, like that of
the Pearly Nautilus in shape, but without any internal partitions,
and having the aperture often expanded and notched behind. The
species of _Maclurea_ (fig. 54) are found both in North America
and in Scotland, and are exclusively confined to the Lower Silurian
period, so far as known. They have the shell coiled into a flat
spiral, the mouth being furnished with a very curious, thick,
and solid lid or "operculum." The Lower Silurian _Pteropods_,
or "Winged snails," are numerous, and belong principally to the
genera _Theca, Conularia_, and _Tentaculites_, the last-mentioned
of these often being extremely abundant in certain strata.
[Illustration: Fig. 52.--_Murchisonia gracilis_, Trenton Limestone,
America. (After Billings.)]
[Illustration: Fig. 53.--Different views of _Bellerophon Argo_,
Trenton Limestone, Canada. (After Billings.)]
[Illustration: Fig. 54.--Different views of _Maclurea crenulata_,
Quebec Group, Newfoundland. (After Billings.)]
[Illustration: Fig. 55.--Fragment of _Orthoceras crebriseptum_,
Cincinnati Group, North America, of the natural size. The lower
figure section showing the air-chambers, and the form and position
of the siphuncle. (After Billings.)]
[Illustration: Fig. 56.--[14] Restoration of Orthoceras, the shell
being supposed to be divided vertically, and only its upper part
being shown. a, Arms; f, Muscular tube ("funnel") by which
water is expelled from the mantle-chamber; c, Air-chambers;
s, Siphuncle.]
[Footnote 14: This illustration is taken from a rough sketch
made by the author many years ago, but he is unable to say from
what original source it was copied.]
Lastly, the Lower Silurian Rocks have yielded a vast number of
chambered shells, referable to animals which belong to the same
great division as the Cuttle-fishes (the _Cephalopoda_), and
of which the Pearly Nautilus is the only living representative
at the present day. In this group of _Cephalopods_ the animal
possesses a well-developed external shell, which is divided into
chambers by shelly partitions ("septa"). The animal lives in
the last-formed and largest chamber of the shell, to which it
is organically connected by muscular attachments. The head is
furnished with long muscular processes or "arms," and can be
protruded from the mouth of the shell at will, or again withdrawn
within it. We learn, also, from the Pearly Nautilus, that these
animals must have possessed two pairs of breathing organs or
"gills;" hence all these forms are grouped together under the
name of the "Tetrabranchiate" Cephalopods (Gr. _tetra_, four;
_bragchia_, gill). On the other hand, the ordinary Cuttle-fishes
and Calamaries either possess an internal skeleton, or if they
have an external shell, it is not chambered; their "arms" are
furnished with powerful organs of adhesion in the form of suckers;
and they possess only a single pair of gills. For this last reason
they are termed the "Dibranchiate" Cephalopods (Gr. _dis_, twice;
_bragchia_, gill). No trace of the true Cuttle-fishes has yet
been found in Lower Silurian deposits; but the Tetrabranchiate
group is represented by a great number of forms, sometimes of
great size. The principal Lower Silurian genus is the well-known
and widely-distributed _Orthoceras_ (fig. 55). The shell in this
genus agrees with that of the existing _Pearly Nautilus_, in
consisting of numerous chambers separated by shelly partitions
(or septa), the latter being perforated by a tube which runs the
whole length of the shell after the last chamber, and is known
as the "siphuncle" (fig. 56, s). The last chamber formed is the
largest, and in it the animal lives. The chambers behind this
are apparently filled with some gas secreted by the animal itself;
and these are supposed to act as a kind of float, enabling the
creature to move with ease under the weight of its shell. The
various air-chambers, though the siphuncle passes through them,
have no direct connection with one another; and it is believed
that the animal has the power of slightly altering its specific
gravity, and thus of rising or sinking in the water by driving
additional fluid into the siphuncle or partially emptying it.
The _Orthoceras_ further agrees with the Pearly Nautilus in the
fact that the partitions or septa separating the different
air-chambers are simple and smooth, concave in front and convex
behind, and devoid of the elaborate lobation which they exhibit
in the Ammonites; whilst the siphuncle pierces the septa either
in the centre or near it. In the Nautilus, however, the shell is
coiled into a flat spiral; whereas in _Orthoceras_ the shell is
a straight, longer or shorter cone, tapering behind, and gradually
expanding towards its mouth in front. The chief objections to
the belief that the animal of the _Orthoceras_ was essentially
like that of the Pearly Nautilus are--the comparatively small
size of the body-chamber, the often contracted aperture of the
mouth, and the enormous size of some specimens of the shell.
Thus, some _Orthocerata_ have been discovered measuring ten or
twelve feet in length, with a diameter of a foot at the larger
extremity. These colossal dimensions certainly make it difficult
to imagine that the comparatively small body-chamber could have
held an animal large enough to move a load so ponderous as its
own shell. To some, this difficulty has appeared so great that
they prefer to believe that the _Orthoceras_ did not live in
its shell at all, but that its shell was an internal skeleton
similar to what we shall find to exist in many of the true
Cuttle-fishes. There is something to be said in favour of this
view, but it would compel us to believe in the existence in Lower
Silurian times of Cuttle-fishes fully equal in size to the giant
"Kraken" of fable. It need only be added in this connection that
the Lower Silurian rocks have yielded the remains of many other
Tetrabranchiate Cephalopods besides _Orthoceras_. Some of these
belong to _Cyrtoceras_, which only differs from _Orthoceras_ in
the bow-shaped form of the shell; others belong to _Phragmoceras_,
_Lituites_, &c.; and, lastly; we have true _Nautili_, with their
spiral shells, closely resembling the existing Pearly Nautilus.
Whilst all the sub-kingdoms of the Invertebrate animals are
represented in the Lower Silurian rocks, no traces of Vertebrate
animals have ever been discovered in these ancient deposits,
unless the so-called "Conodonts" found by Pander in vast numbers
in strata of this age [15] in Russia should prove to be really
of this nature. These problematical bodies are of microscopic
size, and have the form of minute, conical, tooth-shaped spines,
with sharp edges, and hollow at the base. Their original discoverer
regarded them as the horny teeth of fishes allied to the Lampreys;
but Owen came to the conclusion that they probably belonged to
Invertebrates. The recent investigation of a vast number of similar
but slightly larger bodies, of very various forms, in the
Carboniferous rocks of Ohio, has led Professor Newberry to the
conclusion that these singular fossils really are, as Pander
thought, the teeth of Cyclostomatous fishes. The whole of this
difficult question has thus been reopened, and we may yet have
to record the first advent of Vertebrate animals in the Lower
Silurian.
[Footnote 15: According to Pander, the "Conodonts" are found not
only in the Lower Silurian beds, but also in the "Ungulite Grit"
(Upper Cambrian), as well as in the Devonian and Carboniferous
deposits of Russia. Should the Conodonts prove to be truly the
remains of fishes, we should thus have to transfer the first
appearance of vertebrates to, at any rate, as early a period as
the Upper Cambrian.]