THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 9:
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
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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
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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.
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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):—
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GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF WALES.
Fig. 34.
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.
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,
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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:—
GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF NORTH AMERICA.
Fig. 35.
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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
Fig. 36.—Licrophycus Ottawaensis a "Fucoid," from the
Trenton Limestone (Lower Silurian) of Canada. (After Billings.)
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,
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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
Fig. 37.—Astylospongia prœmorsa, cut
vertically so as to exhibit the canal-system in the interior.
Lower Silurian, Tennessee. (After Ferdinand Rœmer.)
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.
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
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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
Fig. 38.—A small and perfect specimen of Stromatopora
rugosa, of the natural size, from the Trenton Limestone of
Canada. (After Billings.)
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.
Passing next to the sub-kingdom of Cœlenterate animals
(Zoophytes, Corals, &c.), we find that this great group, almost
or wholly absent in the Cambrian, is represented in Lower
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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
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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.
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
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real in Bohemia, they are exclusively confined to strata of Lower
Silurian age, and are not known to occur in the Upper Silurian.
Fig. 40.—Central portion of the colony of Didymegraptus
divaricatus, Upper Llandeilo, Dumfresshire. (Original.)
Lastly, there is a group of Graptolites (Phyllograptus, fig.
42) in which the colony is leaf-like in form, and is composed
Fig. 41.—Examples of Diplograptus pristis,
showing variations in the appendages at the base. Upper Llandeilo,
Dumfriesshire. (Original.)
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.
of four rows of cells springing in a cross-like
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manner from the common stem. These forms are highly
characteristic of the Arenig group.
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 Cœlenterate 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,
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but united together in different
ways into a common mass. Simple corals, therefore, are the
Fig. 43.—Zaphrentis Stokesi, a simple "cup-coral,"
Upper Silurian, Canada. (After Billings.)
Fig. 44.—Upper surface of a mass of Strombodes
pentagonus. Upper Silurian, Canada. (After Billings.)
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.
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).
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
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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
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.)
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
Chœtetes, 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.
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
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jointed together; and the stem was made up of numerous distinct
pieces or joints, flexibly united to each other by membrane. The
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.)
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
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in later periods, they are pre-eminently characteristic of the
earlier portion of the Palæozoic
epoch.
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
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.)
Crustaceans. Amongst these are numerous little bivalved
forms—such as species of Primitia (fig. 47, f),
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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, Illœnus, 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.
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"
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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
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.)
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.)
different forms of Brachiopods which present themselves in the
Lower Silurian deposits; but we may select the three genera
Orthis, Strophomena, and Leptœna for
illustration, as being specially characteristic of this period,
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.)
though not exclusively confined to it. The numerous shells which
belong to the extensive and cosmopolitan genus Orthis (fig.
50, a, b, c,
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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,
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'', Leptœna sericea, Llandeilo and Caradoc, Europe
and America. (After Meek, Hall, and the Author.)
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 Leptœna,
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.
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
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Modiolopsis, Ctenodonta, Orthonota, Palœarca, Lyrodesma,
Fig. 52.—Murchisonia gracilis, Trenton Limestone,
America. (After Billings.)
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.
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
Fig. 53.—Different views of Bellerophon Argo,
Trenton Limestone, Canada. (After Billings.)
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
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it is organically connected by muscular attachments. The head is
furnished with long muscular processes or "arms," and can be
Fig. 54.—Different views of Maclurea crenulata,
Quebec Group, Newfoundland. (After Billings.)
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
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further agrees with the Pearly Nautilus in the fact that
the partitions or septa separating the different air-chambers are
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.)
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.
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
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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.
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